Test Bank for Janeway’s Immunobiology, 9th Edition By Kenneth Murphy, Casey Weaver
JANEWAY'S IMMUNOBIOLOGY, 9
CHAPTER 2: INNATE IMMUNITY: THE FIRST LINES OF DEFENSE
Anatomic barriers and initial chemical defenses
2-1 Infectious diseases are caused by diverse living agents that replicate in their hosts
2.1 Multiple choice: Antibodies, complement proteins, and phagocytic cells provide
effective protection against all of the following types of infections in Figure Q2.1, except:
2.2 Multiple choice: Pathogenic infections induce damage to the host by a variety of
mechanisms. While many mechanisms are direct effects of the pathogen, some
damaging mechanisms result from the immune response to the infection, as illustrated in
Figure Q2.2. Examples of damage caused by the host immune response are:
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2.3 True/False: Mucosal surfaces and external epithelia are major routes of pathogenic
infection. Mucosal surfaces are found in tissues such as the gastrointestinal tract, the
reproductive tract and the mouth and respiratory tract. While the mouth and respiratory
tract are routes of virus but not bacterial infections, the gastrointestinal tract is the route
for bacterial but not virus infections.
2-2 Epithelial surfaces of the body provide the first barrier against infection
2.4 Multiple choice: Epithelial surfaces provide the first line of defense against infection by
the use of several types of mechanisms. One of the chemical mechanisms used by
A. Joining of epithelial cells by tight junctions
B. Secretion of antimicrobial peptides by epithelial cells
C. Production of mucus, tears, or saliva in the nose, eyes, and oral cavity
D. Movement of mucus by cilia
E. Peristalsis in the gastrointestinal tract
2.5 Multiple choice: Women with urinary tract infections caused by E. coli are generally
treated with a course of antibiotics. A common complication of the antibiotic treatment is
the occurrence of a vaginal yeast infection caused by Candida albicans, an organism
that is normally present in very low numbers in the human vaginal tract. This
complication occurs because:
A. The E. coli infection damages the reproductive epithelium, causing a breach in
the tight junctions and allowing invasion by the Candida albicans.
B. The E. coli infection induces adhesion molecule expression on the reproductive
epithelium, allowing attachment of the yeast.
C. The antibiotic treatment kills all strains of fungi present in the reproductive tract,
except the Candida albicans.
D. The E. coli infection causes gastrointestinal distress leading to diarrhea.
E. The antibiotics kill many of the commensal organisms in the reproductive tract,
allowing overgrowth of the fungus.
2-3 Infectious agents must overcome innate host defenses to establish a focus of
2.6 Short answer: Our environment contains masses of microorganisms, many of which
reside as commensal organisms on our body’s mucosal and epithelial surfaces without
causing disease. What two features distinguish a pathogenic microbe from these
2-4 Epithelial cells and phagocytes produce several kinds of antimicrobial proteins
2.7 Multiple choice: Streptococcus pneumoniae is a Gram-positive bacterium that
colonizes the mucosal surface of the upper respiratory tract in humans. The presence of
this bacterium in the nose and throat is widespread in the population, and in most
people, colonization with Strep. pneumoniae is asymptomatic. Figure Q2.7 shows a
comparison of in vitro growth curves of the wild-type strain of Strep. pneumoniae, as well
as a Strep. pneumoniae mutant strain with a defect in one bacterial gene. The graph on
the right shows the growth curve following addition of lysozyme during the logarithmic
phase of bacterial growth.
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Which statement could account for the data in these graphs?
A. Strain B is wild-type Strep. pneumoniae, and strain A is a mutant that cannot
modify its peptidoglycan to be lysozyme-resistant.
B. Strain B is wild-type Strep. pneumoniae, and strain A is a mutant that that
expresses increased levels of LPS.
C. Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that cannot
modify its peptidoglycan to be lysozyme-resistant.
D. Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that secretes an
enzyme that inactivates lysozyme.
E. Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that cannot
grow well in vitro.
2.8 Multiple choice: The production of antimicrobial peptides is one of the most
evolutionarily ancient mechanisms of defense for multicellular organisms, and most
eukaryotic species make many different forms of these proteins. For instance, human
paneth cells in the gastrointestinal epithelium make 21 different defensins. The reason
for this diversity of antimicrobial peptides is:
A. Epithelial cells make different forms than those made by neutrophils.
B. Neutrophils make many different defensins and store them as inactive proteins in
their secretory granules.
C. Most of them are produced only in response to infection.
D. The production of different peptides is induced following a bacterial infection
versus a fungal infection.
E. Each one has distinct activities against Gram-negative bacteria, Gram-positive
bacteria, or fungi.
2.9 True/False: Neutrophils regulate the production of active cathelicidins (a class of
antimicrobial peptides) by segregating the inactive propeptide from the processing
enzyme that cleaves and activates it in two different types of cytoplasmic granules.
These two types of granules are induced to fuse with phagosomes after ingestion of
microbes, bringing the processing enzyme and the propeptide together.
The complement system and innate immunity
2-5 The complement system recognizes features of microbial surfaces and marks them
for destruction by coating them with C3b
2.10 Multiple choice: Although the complement cascade can be initiated by antibodies
bound to the surface of a pathogen, complement activation is generally considered to be
an innate immune response. This is because:
A. Two of the three pathways for complement activation are initiated by
constitutively produced recognition molecules that directly interact with microbial
B. When the complement cascade leads to the formation of a membrane-attack
complex, the pathogen is killed.
C. Several of the soluble products generated by complement activation lead
promote the inflammatory response.
D. Complement proteins bound to the pathogen promote uptake and destruction by
E. The C3 convertase is only produced when complement activation is initiated by
antibody binding to a pathogen.
2.11 Multiple choice: The formation of the C3 convertase is a key step in complement
activation that occurs in all three complement pathways. This enzyme cleaves C3 in
blood plasma, leading to a conformational change in the C3b fragment that exposes its
reactive thioester group. The activated C3b is potentially harmful to the host, if it
becomes covalently attached to a host cell, rather than to the surface of a pathogen.
This deleterious outcome is largely avoided by:
A. The inability of active C3b to diffuse away in the blood plasma.
B. The inability of active C3b to covalently attach to the membranes of eukaryotic
C. The rapid hydrolysis of active C3b in solution, rendering it inactive.
D. The tight binding of active C3b to the C3 convertase.
E. The ability of active C3b to recruit phagocytic cells.
2-6 The lectin pathway uses soluble receptors that recognize microbial surfaces to
activate the complement cascade
2.12 Short answer: Infants and young children with deficiencies in specific complement
components present with recurrent respiratory infections caused by extracellular
bacteria. The peak age of susceptibility is between 6 and 12 months after birth. At this
time, as shown in Figure Q2.12, maternal antibodies acquired by the child during fetal
gestation are nearly gone, but the child is not yet generating robust antibody responses
to new infections, as indicated by the low circulating levels of IgG and IgA. As children
with this immunodeficiency get older, they outgrow this disease and show no further
evidence of these recurrent infections. Based on this information, name one likely gene
deficiency (in the complement system) that could cause this primary immunodeficiency,
and the specific complement pathway likely to be affected. Explain your answer.
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2.13 Multiple choice: Mannose binding lectins (MBL) and ficolins are the two classes of
proteins that can initiate the lectin pathway of complement activation. These proteins are
selective for activating complement on the surfaces of microbial pathogens rather than
host cells because:
A. Their higher-order oligomeric structure can be assembled only after the
monomers first bind to pathogen membranes.
B. They only recruit MASP (MBL-associated serine proteases) proteins when bound
to pathogen surfaces and not when bound to host cells.
C. They only undergo the conformational change needed to activate MASP proteins
when bound to a pathogen and not when bound to a host cell.
D. They only bind to carbohydrate side chains and oligosaccharide modifications
found on pathogen surfaces but not on host cell membranes.
E. The activated MASP proteins are rapidly inactivated by hydrolysis when present
on the surface of a host cell.
2-7 The classical pathway is initiated by activation of the C1 complex and is homologous
to the lectin pathway
2.14 Multiple choice: The classical complement pathway is initiated by C1q binding to the
surface of a pathogen. In some cases, C1q can directly bind the pathogen, for instance
by recognizing proteins of bacterial cell walls, but in most cases C1q binds to IgM
antibodies that are bound to the pathogen surface. How does this IgM-binding feature of
C1q contribute to rapid, innate immune responses rather than to slow, adaptive
A. C1q induces B lymphocytes to begin secreting antibody within hours of pathogen
B. Natural antibody that binds to many microbial pathogens is produced prior to
C. C1q binds to C-reactive protein which then binds to IgM on the pathogen surface.
D. C1q directly induces inflammation, recruiting phagocytes and antibodies from the
blood into the infected tissue.
E. C1q binds to dendritic cells in the infected tissue, inducing them to secrete
2.15 True/False: The classical and lectin pathways of complement activation converge at the
step of C3 activation. However, the initiating steps of each pathway use protein
components and enzymatic mechanisms that share no similarity with each other.
2-8 Complement activation is largely confined to the surface on which it is initiated
2.16 Multiple choice: Opsonization of pathogens by both antibodies and complement
proteins (C3b) leads to uptake and destruction of the pathogen by phagocytic cells that
express both Fc receptors and complement receptors. Which of the following in Figure
Q2.16 is the most efficient form of dual opsonization of the pathogen by antibody and
C3b to maximize phagocytosis?
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2-9 The alternative pathway is an amplification loop for C3b formation that is accelerated
by properdin in the presence of pathogens
2.17 Multiple choice: The alternative pathway of complement activation has an important
role in innate immunity, due to its ability to greatly amplify the amount of C3b deposited
onto the pathogen surface. This amplification occurs because:
A. The C3 convertase of the alternative pathway is much more active than those of
the classical and lectin pathways.
B. The C3 convertase of the alternative pathway works as a soluble enzyme in the
C. The C3 convertase of the alternative pathway cannot be inactivated by
complement regulatory factors in the host.
D. The C3 convertase of the alternative pathway is more efficiently recruited to
pathogen surfaces than the C3 convertases of the classical and lectin pathways.
E. The C3 convertase of the alternative pathway contains C3b, and can generate
more of itself.
2.18 True/False: The C3 convertase of the alternative complement pathway amplifies the
overall magnitude of complement activation regardless of which of the three pathways
initiated the complement activation initially.
2.19 Multiple choice: Patients with recurrent infections of Neisseria meningitidis, an
extracellular bacterial pathogen that causes meningitis, were examined to determine the
underlying cause of their immunodeficiency. A subset of these patients were found to
have defects in complement activation on the bacterial surface, a process that for this
bacterium is dominated by alternative complement activation leading to C3b deposition
on the pathogen surface. When neutrophils from these patients were examined in vitro,
the results in Figure Q2.19 were obtained.
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Based on these data, the identity of the green neutrophil mediator in Figure Q2.19 is
likely to be:
A. Complement factor B
B. The C3 convertase
C. Factor P (properdin)
E. Mannose-binding lectin (MBL)
2-10 Membrane and plasma proteins that regulate the formation and stability of C3
convertases determine the extent of complement activation
2.20 Multiple choice: One form of anemia results when individuals have a deficiency in the
enzyme phosphatidylinositol glycan A (PIGA). This enzyme is required for the
membrane attachment of proteins anchored by glycolipids to the plasma membrane,
using what is called a ‘GPI-linkage.’ Included in the group of GPI-linked cell surface
proteins is DAF/CD55. These individuals become anemic because:
A. DAF/CD55 prevents the lysis of red blood cells by infecting pathogens.
B. DAF/CD55 normally prevents the spleen from clearing healthy red blood cells
from the circulation.
C. In the absence of PIGA, the red blood cell membrane is bare of proteins allowing
increased access of complement activating proteins to attach to the cell
D. DAF/CD55 is a complement inhibitory protein that inactivates any C3 convertase
that may form on host cell surfaces.
E. In the absence of PIGA, red blood cells are unable to synthesize high levels of
2-11 Complement developed early in the evolution of multicellular organisms
2.21 Short answer: In vertebrates, complement activation generally involves a pathogen
recognition step followed by a proteolytic cascade that produces the effector proteins
that function in opsonization, membrane attack, and inflammation.
a) Which of these is likely to be the most evolutionarily primitive aspect of the
b) Which pathway of complement initiation is likely to be the one that most recently
2-12 Surface-bound C3 convertase deposits large numbers of C3b fragments on
pathogen surfaces and generates C5 convertase activity
2.22 True/False: The C3 convertase amplifies the process of complement activation by
generating large amounts of C3b and cleaving large numbers of C5 molecules.
2-13 Ingestion of complement-tagged pathogens by phagocytes is mediated by
receptors for the bound complement proteins
2.23 Multiple choice: Even when the complement cascade fails to proceed beyond
generating the C3 convertase, complement activation is effective at inducing pathogen
uptake and destruction. This process of immune protection is mediated by:
A. Activation of complement inhibitory receptors on phagocytes that promote
B. Activation of soluble proteases in the serum that disrupt pathogen membranes
C. Engagement of complement receptors on phagocytes by C3b and its cleavage
products which promotes phagocytosis
D. Engagement of complement receptors on B cells that promotes antibody
E. Stimulation of antimicrobial peptide secretion by phagocytes
2.24 Multiple choice: B cells express a complement receptor that binds to C3b cleavage
products, such as iC3b and C3dg. When a B cell with an antigen receptor that
specifically recognizes that pathogen also has its complement receptor stimulated
because the pathogen is opsonized with these C3 fragments, B cell activation is greatly
enhanced. Due to this mechanism, B cells can be activated by much lower
concentrations of antigen (in this case, the pathogen) than if the antigen is devoid of
complement components. This mechanism functions to:
A. Ensure that pathogens are readily detected by the adaptive immune system
before they replicate to high levels in the host
B. Prevent B cells from being activated in response to antigens that are not
C. Allow B cells to phagocytose the pathogen and help destroy it
D. Induce increased rounds of B cell replication to make more pathogen-specific B
E. Allow the B cell to block pathogen replication by interfering with multiple
pathogen surface functions
2-14 The small fragments of some complement proteins initiate a local inflammatory
2.25 Short answer: Recent studies using mouse models of pulmonary inflammation (a model
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for human asthma) have found that mice deficient in the C3a receptor have greatly
reduced disease symptoms when challenged with inhaled preparations containing
extracts of the fungal pathogen Aspergillus fumigatus. Specifically, the C3a receptordeficient
2-15 The terminal complement proteins polymerize to form pores in membranes that can
kill certain pathogens
2.26 Multiple choice: The terminal components of the complement pathway assemble to
form a membrane attack complex that can induce pathogen lysis and death. Yet,
evidence indicates that this feature of complement is less important than the earlier
steps that promote pathogen opsonization and induce inflammation. This conclusion is
A. In vitro experiments showing that very few species of bacteria are susceptible to
lysis by the membrane attack complex
B. Experiments indicating that only bacteria, but not viruses or fungi, are susceptible
to lysis by the membrane attack complex
C. The very low levels of terminal complement components in the serum
D. The fact that other mammalian species lack the terminal components of the
complement pathway needed to form the membrane attack complex
E. The limited susceptibility to infections of patients with deficiencies in terminal
2-16 Complement control proteins regulate all three pathways of complement activation
and protect the host from their destructive effects
2.27 Multiple choice: Multiple pathways for regulating complement activation limit the
potential damage caused by complement deposition on host cells or caused by the
spontaneous activation of complement proteins in the plasma. Genetic deficiencies in
these mechanisms often lead to chronic inflammatory diseases, but in some cases can
paradoxically lead to increased susceptibility to bacterial infections. This latter outcome
may occur because:
A. Complement regulatory proteins have dual functions in inhibiting and promoting
B. Uncontrolled complement activation leads to the depletion of serum complement
C. The inhibition of the membrane attack complex by complement regulatory
proteins normally leads to enhanced activation of the early steps of the
D. Complement regulatory proteins normally cause the rapid depletion of plasma
E. Uncontrolled complement activation recruits the majority of phagocytic cells,
leaving few remaining to fight infections in the tissues.
2.28 Short answer: Although homozygous deficiencies in complement regulatory proteins
cause serious diseases, more subtle alterations in the balance of complement activation
versus inhibition have been found to contribute to disease susceptibility. Describe the
genetic evidence linking subtle alterations in complement regulatory proteins to disease
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2-17 Pathogens produce several types of proteins that can inhibit complement activation
2.29 Multiple choice: The importance of complement activation as an innate immune
defense against infections is illustrated by:
A. The evolution of complement avoidance strategies by many pathogens
B. The large number of proteins involved in the complement pathway
C. The large number of complement regulatory pathways expressed by the host
D. The existence of three different mechanisms for initiating complement activation
E. The ability of the membrane attack complex to lyse some pathogens
2.30 True/False: Several pathogens produce proteins, either membrane-bound or secreted,
that inactivate C3b that might be deposited on the pathogen surface. C3b is specifically
targeted due to its central position in all three complement pathways.
2.31 Synthesis question: Four different clinical isolates of the Gram-positive bacterium,
Staphylococcus aureus, are tested for their abilities to resist innate immune defense
mechanisms. For these experiments, each bacterial strain is first grown in culture to
achieve log-phase replication, and then cultures are supplemented with dilutions of
human serum containing normal serum proteins as well as antibodies capable of binding
to S. aureus. One hour later, the cultures are analyzed and the numbers of live bacteria
are quantitated. The data from this experiment are shown in Figure Q2.31A.
a) From these data, what general conclusions can be reached about the four strains of
To identify the bactericidal mechanisms killing each strain of S. aureus, the serum is
depleted of complement C3 by running it over an anti-complement C3 antibody affinity
column. The experiment above is then repeated and the data are shown in Figure
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b) What is the most likely mechanism accounting for the killing of strain D in this
To determine whether strains A and C are susceptible to the same microbicidal pathway,
the serum is depleted of antibody by running over an anti-human immunoglobulin affinity
column. Following this treatment, it is found that strain A, but not strain C is still killed by
incubation with the serum.
c) From these data, what is the most likely mechanism killing S. aureus strain A? What
about strain C?
2.32 Synthesis question: Pseudomonas aeruginosa is a Gram-negative bacterium that
causes severe, and often life-threatening infections in immunocompromised individuals.
In susceptible individuals, P. aeruginosa can establish infections in a wide range of
tissues, including the lung, the GI tract, the eye, the ear, the urinary tract, the skin, and
the blood. This bacterium is common in the environment, and is found on the skin of
approximately 5% of healthy individuals. It is often found on hospital equipment, such as
ventilators and catheters, and as a consequence, P. aeruginosa accounts for ~10% of
hospital-acquired infections. To study the role of complement in the early innate immune
response to P. aeruginosa, the following studies in mice were performed. Mice deficient
in complement C3 or C5 (C3-/- or C5-/-, respectively) were infected by intratracheal
inoculation with 10
colony forming units (CFU) of P. aeruginosa, and survival was
monitored over the first 72 hrs post-infection. The data from these studies are shown in
Figure Q2.32. Genetic data from human population studies also indicate that Individuals
with genetic deficiencies in one of the collectins or ficolins show increased susceptibility
to P. aeruginosa infections.
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a) Based on these data, evaluate the importance of complement in protection against P.
aeruginosa infection, and describe the most likely complement pathway(s) involved in
pathogen recognition and in pathogen destruction.
Another group of individuals that are highly susceptible to P. aeruginosa infections are
patients with the disease cystic fibrosis. These individuals suffer from the production of a
thick mucus secretion in their lungs, which clogs the bronchial tubes. A similar increase
in viscosity of bodily secretions is seen in these patients’ sweat, digestive fluid, and
gastrointestinal mucus. In these patients, the most common form of lung infection is that
of P. aeruginosa.
b) What is the most likely explanation for the increased susceptibility of cystic fibrosis
patients to P. aeruginosa and other infections?
All extracellular forms of pathogens are targets for antibodies, complement, phagocytic cells and
antibody-dependent immune clearance mechanisms. Once a pathogen, such as a virus or
intracellular protozoan, invades a cell and begins replicating in the cell, these mechanisms are
no longer able to clear the infection. These intracellular stages of pathogenic infection require
cellular responses, such as those mediated by T cells or NK cells.
Pathogens cause direct tissue damage by the production of exotoxins or endotoxins, as well as
by direct cytopathic effects. Tissue damage caused by the host immune response include
damage caused by cell-mediated immunity and by the accumulation of immune complexes.
Both bacterial and virus infections can use both the mouth and respiratory tract and the
gastrointestinal tract. There is no route of infection that is specific for a single category of
Anti-microbial peptides are produced by epithelia at all mucosal and epidermal surfaces. These
chemicals are important in immune protection against microbial pathogens. All other choices are
mechanical mechanisms by which surface epithelia protect against infections, not chemical
Commensal organisms associated with all epithelial surfaces provide protection against
colonization by pathogenic microbes. One mechanism is by competition for nutrients as well as
for attachment sites on epithelial surfaces. Another mechanism is by producing metabolites that
are toxic to other organisms. When these commensal microorganisms are eliminated by
antibiotic treatment, pathogenic microbes are able to step into the void and establish an
2.6: The first important feature of a pathogenic microbe is that it must establish a replicating
colony of organisms in our body. This can occur by the pathogen crossing an epithelial barrier
and replicating in the tissue, or by attaching to the epithelial surface and establishing a colony
there. The second feature is that the pathogen needs to have special mechanisms to evade the
innate immune response.
Strep. pneumoniae is a Gram-positive bacterium that readily colonizes the human nose and
mouth, due to the resistance of its peptidoglycan to degradation by lysozyme, an enzyme that is
abundant in tears, saliva, and mucus. Wild-type Strep. pneumoniae is naturally lysozymeresistant
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The diversity of antimicrobial peptides is a reflection of the diversity of microbial pathogens that
they attack. Some antimicrobial peptides are active against Gram-negative bacteria, while
others are only active against Gram-positive bacteria. Other antimicrobial peptides are only
active against fungal pathogens, and some are able to disrupt the membrane envelopes of
All antimicrobial peptides, including cathelicidins, are produced as inactive propeptides. The
active forms of the peptides are generated following proteolytic cleavage of the propeptides.
Neutrophils constitutively produce cathelicidins, which are synthesized as inactive propeptides.
The inactive cathelicidin propeptides are stored in secondary granules, whereas the cleavage
enzyme, neutrophil elastase, is stored in primary granules. These two types of granules are
induced to fuse with phagocytic vesicles, called phagosomes, after the neutrophil has engulfed
a pathogen. This fusion brings the cleavage enzyme together with the cathelicidin propeptide,
leading to cathelicidin activation.
There are three pathways for initiating complement activation. One of them, known as the
classical pathway, occurs when the pathogen has antibodies bound to its surface, leading to
recruitment of C1q. The other two pathways, the lectin pathway and the alternative pathway, are
initiated by mechanisms that do not require antibodies directed against the pathogen surface.
These latter two pathways are dependent on constitutively produced, and therefore ‘innate’
recognition molecules that directly bind to pathogen surfaces, initiating complement activation.
Active C3b is highly labile, and is rapidly inactivated by hydrolysis. This prevents the C3b from
remaining active should it diffuse away from the pathogen surface where it was activated by the
2.12: MBL or MASP.
Infants and small children with defects in MBL or MASP show recurrent upper respiratory
infections by extracellular bacteria. This is due to a defect in the lectin pathway of complement
activation. When maternal antibodies wane and the child is not yet generating robust antibody
responses on its own, complement activation cannot proceed by the classical pathway. During
this time, protection against upper respiratory bacterial infections is highly dependent on the
lectin pathway, initiated by MBL or collectin binding to the pathogen. The information provided
rule out the alternative pathway, which is initiated by spontaneous C3 cleavage. If there was a
defect in C3, or a downstream component of the complement cascade shared by all three
pathways, the recurrent infections would not disappear as children age.
MBL and ficolins have binding specificity for carbohydrate side chains and oligosaccharide
modifications that are unique to microbial pathogens, and not found on host cells. MBL binds to
mannose, fucose, and GlcNac residues, which are common on microbial glycans; in contrast,
MBL does not bind to sialic residues, which terminate vertebrate glycans. Ficolins have
specificity for binding to oligosaccharides containing acetylated sugars, a structure also only
found on pathogen surfaces, not on host cells.
Natural antibody, which is primarily of the IgM class, is produced in the body prior to pathogen
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exposure. These antibodies are widely reactive with many microbial pathogens, although they
generally have low affinity for the pathogen. However, since IgM is a pentamer of IgM
monomers, each IgM pentamer has 10 binding sites for antigen, allowing even low affinity
antibodies to bind, due to the increased avidity of multiple binding sites. This natural antibody
will then recruit C1q, leading to complement activation. Since the natural antibody pre-exists
prior to pathogen exposure, this response is rapid and is considered part of the innate immune
The initiating steps of the classical and lectin pathways of complement activation are remarkably
conserved in their mechanisms. The pathogen recognition component of the classical pathway,
C1q, has structural similarity to MBL and the ficolins. The C1r and C1s components of the
classical pathway, that are activated to form the serine protease, are closely related to the
MASP proteins of the lectin pathway.
The most efficient form of opsonization by antibody plus C3b is when the complement protein is
covalently linked to the antibody molecule. This leads to efficient engagement of both Fc
receptors and complement receptors on phagocytic cells.
The C3 convertase (C3bBb) of the alternative pathway contains C3b, allowing it to generate
more of itself and amplify the overall level of C3b formed. Once additional molecules of C3b are
made by C3bBb, these can recruit additional molecules of factor B and the plasma protease
factor D. Factor D cleaves factor B, and one of the products, Bb, remains associated with C3b,
forming more active C3 convertase.
The C3 convertase of the alternative pathway contains C3b, allowing it to generate more of itself
and amplify the overall level of C3b formed. Since C3b is a common intermediate for all three
pathways of complement activation, once the initial C3b is generated by any of the pathways,
the recruitment of factor B, and cleavage by factor D can proceed. By this mechanism, the initial
C3b generated forms an amplification loop leading to more C3b, regardless of how the initial
C3b was made.
Factor P (properdin) is made by neutrophils and stored in their granules. When neutrophils are
activated by the presence of pathogens, factor P is released. Factor P binds to and stabilizes
the reactive form of C3 (C3-H
0) and the C3 convertase C3bBb. In the absence of factor P, the
alternative complement pathway is inefficient, due to the rapid spontaneous inactivation of C3H
0 and C3bBb. This pathway is particularly important in protection against Neisseria
meningitidis, and patients that are deficient in producing factor P are highly susceptible to
infections with this pathogen.
Host cells express several complement-regulatory proteins on their surface. These proteins
function to rapidly inactivate any C3bBb (active C3 convertase) that may form on the host cell
membrane. Several of these complement regulatory proteins use GPI-linkages to attach to the
host cell membrane. Included in this group is DAF/CD55, which competes with factor B for
binding to C3b on the cell surface, and displaces Bb from any active C3 convertase that has
already formed. The absence of DAF/CD55 makes host cells susceptible to complement-
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mediated lysis. For reasons that are not entirely clear, red blood cells are particularly
susceptible to complement-mediated lysis and the absence of the GPI-linked subset of
complement regulatory proteins is sufficient to cause red blood cell lysis leading to anemia.
a) The most primitive form of a complement system is one that resembles our alternative
complement pathway, with ancestral homologs of C3 and factor B that make a C3
convertase. This provides a mechanism for opsonizing infecting bacteria and increasing
their phagocytosis by phagocytic cells. These ancestral homologs of C3 and factor B
have been found in echinoderms, and may even have existed in even more primitive
organisms such as corals and sea anemones.
b) The latest evolutionary development in the complement system is the classical pathway,
which makes use of antibody binding to initiate complement activation. The adaptive
immune system, including the production of antibodies, is only found in vertebrates.
The C3 convertase does generate large numbers of C3b molecules which become attached to
the pathogen surface in the vicinity of the convertase. This enzyme can only cleave C5 when
bound to a molecule of C3b, generating the C5 convertase. The generation of the C5
convertase occurs at a much lower level than the C3 convertase, and many fewer molecules of
C5 than C3 are cleaved.
Phagocytes have a variety of receptors that recognize C3b and fragments of C3b, such as iC3b.
Engagement of these complement receptors stimulates phagocytosis of the C3b-coated
pathogen, leading to pathogen destruction.
The complement receptor on B cells, CD21, is often referred to as the B cell co-receptor. When
this receptor is engaged together with the B cell antigen receptor, the B cell can be activated by
much lower concentrations of antigen compared to antigen lacking ligands for CD21.
Experiments have indicated that CD21 stimulation can reduce the concentration of antigen
needed to activate the B cell by 100–1000-fold. This allows B cells to detect small numbers of
infecting pathogens, to initiate an adaptive response prior to the occurrence of a high pathogen
load in the host.
2.25: When complement is activated in the lung in response to the inhaled preparations of the
fungus, the C3 convertase generates C3a. C3a induces a local inflammatory response in the
lung, by acting on the vascular endothelial cells. This response includes increased vascular
permeability, leading to an increase of fluid in the lung, and also acts to up-regulate adhesion
molecules on the local vascular endothelium. As a result, there is increased recruitment of
granulocytes, monocytes, and lymphocytes into the lung.
Patients with genetic deficiencies in terminal complement components show only a limited
increase in susceptibility to infection. These individuals are more susceptible to infection by
Neisseria species that cause gonorrhea or meningitis. Otherwise, these individuals show no
other increased susceptibility to infection, indicating that formation of the membrane attack
complex is a less important aspect of complement activation compared to the earlier steps that
lead to opsonization of the pathogen as well as inducing inflammation.
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Individuals with a genetic defect in factor I are subject to recurrent infections with pyogenic (pusforming)
Strain A and strain C are equally sensitive to a serum component (or set of components)
that can kill S. aureus. Strain B is resistant to all possible serum components that can kill
S. aureus. Strain D is also sensitive to a serum component(s) that can kill S. aureus, but
is killed by a different mechanism than the one(s) killing strains A and C. This latter
conclusion is based on the observation that the factor(s) killing strain D are no longer
active when the serum is diluted 1:16, whereas the activities that kill strains A and C are
still active at this dilution.b) Strains A and C are no longer killed when complement C3
is depleted from the serum. This indicates that these two strains are susceptible to
complement-mediated lysis. The killing of strain D is not affected by depletion of C3,
indicating a distinct mechanism. Strain D is most likely being killed by defensins, or
another antimicrobial peptide. It is unlikely that strain D is being killed by lysozyme in
these experiments, as lysozyme is predominantly found in tears and saliva, rather than
c) Since strain A is still killed following depletion of antibody, but not following depletion of
C3, it is likely that strain A is susceptible to the lectin pathway of complement activation,
initiated by binding of ficolin (or collectin) to the bacterial surface. Ficolins are the most
likely initiators of complement activation in this case, as they are at high concentrations
in the serum. Although it is theoretically possible that strain A is killed by the alternative
pathway of complement activation, this possibility is unlikely. The spontaneous
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hydrolysis of C3 produces a fluid-phase C3 convertase that is very short-lived. This fluidphase
Strain C is no longer killed following depletion of antibody. Therefore, strain C is likely
susceptible to the classical pathway of complement activation.
a) Complement is essential for early protection against this dose of P. aeruginosa in mice.
In the absence of either C3 or C5, all of the infected mice succumb to the infection by 2
days post-infection. Based on the human population studies indicating that individuals
deficient in collectins or ficolins are also more susceptible to this bacterial infection, it is
likely that the lectin pathway of complement activation is the primary mechanism for
initiating the complement cascade. Since C5 is required, this points to an important role
for the membrane attack complex in pathogen destruction. However, it is also possible
that the requirement for C5 is due to the role of C5a in promoting inflammation, leading
to the recruitment of phagocytic cells to the site of infection. In this latter case, pathogen
destruction would be due to uptake by phagocytic cells.
b) Due to their defect in regulating the viscosity of bodily secretions, the mechanical and
chemical mechanisms of immune resistance at the body’s epithelial barriers do not
function normally. Instead of serving as a fluid that flushes away potential pathogens at
these barriers, the mucus of cystic fibrosis patients functions as a stagnant reservoir
ideal for bacterial colonization. Furthermore, the thick mucus prevents the normal
diffusion of antimicrobial enzymes and peptides that are secreted into the mucus.
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