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Abbas: Basic Immunology, 4
Chapter 02: Innate Immunity
1. Which of the following statements about the innate immune system is NOT true?
A. Innate immunity is present in all multicellular organisms, including plants and
B. Deficiencies in innate immunity markedly increase host susceptibility to infection,
even in the setting of an intact adaptive immune response.
C. Innate immunity is better suited for eliminating virulent, resistant microbes than is
D. The innate immune response can be divided into recognition, activation, and
E. The innate immune response against microbes influences the type of adaptive
immune response that develops.
Innate immunity is the first line of defense against infections, yet many pathogenic
microbes have evolved strategies to resist innate immunity. Adaptive immunity, being
more potent and specialized, plays a critical role in defending against these virulent
microbes. Innate immunity is the phylogenetically oldest mechanism of microbial
defense, and it is present in all multicellular organisms, including plants and insects.
Studies have shown that hampering effector mechanisms of innate immunity renders
hosts much more susceptible to infection, even with a functional adaptive immune
system. It is also true that, like the adaptive response, the innate immune response
consists of recognition, activation, and effector phases. Although it provides the initial,
rapid response against microbes, innate immunity can influence adaptive immune
responses to tailor them against particular microbes.
2. A 4-year-old girl stepped on a rusty nail in her backyard. Two days later, she is taken
to the pediatrician because her heel is painful, red, and swollen and is warm to the touch.
All of the following are mechanisms of innate immunity that may be protecting the
patient against pathogenic microbes in the heel wound EXCEPT:
A. Epithelial barrier function of the skin of her foot
B. Intraepithelial lymphocytes present in the skin
C. Circulating neutrophils migrating to the site of the wound
D. Soluble cytokines that induce a local inflammatory response
E. Circulating anti-tetanus toxin antibodies
Secreted antibodies against protein antigens are effectors of humoral immunity, a
component of the adaptive immune system. All other mechanisms listed are part of the
innate immune system. Intact epithelial surfaces prevent microbial entry, and epithelial
cells express anti-microbial factors, such as defensins. Neutrophils are effector cells that
function in early phagocytosis and killing of microbes. Cytokines that mediate
inflammation (e.g., tumor necrosis factor, interleukin-1, chemokines) are components of
innate immunity. Intraepithe-lial T lymphocytes present in the epidermis and mucosal
epithelia express a limited diversity of antigen receptors; as such, they are considered
effector cells of innate immunity and function in host defense by secreting cytokines,
activating phagocytes, and killing infected cells.
3. Which of the following comparisons of the innate and adaptive immune systems is
A. The innate immune system is more likely to recognize normal self, and therefore
cause autoimmunity, than is the adaptive immune system.
B. Receptors used for recognition in innate immunity are encoded in the germline,
whereas those of the adaptive immune system are encoded by genes generated via
somatic recombination of germline receptor gene loci.
C. The innate and adaptive immune systems share some of the same effector
D. Both the innate and adaptive immune systems can recognize nonmicrobial
E. The innate immune system does not have memory but the adaptive immune
Innate immune system receptors are encoded by germline genes that have evolved to
recognize microbial structures or molecules produced by stressed self, and therefore there
is little chance of innate immune responses to normal self. Because the specificities of
adaptive immune system receptors (Ig or T cell receptor molecules) are randomly
generated by somatic recombination and junctional-diversity mechanisms, there is a
greater chance that the adaptive immune system receptors may recognize normal self
molecules, leading to autoimmunity. Mechanisms of tolerance minimize this possibility,
but these mechanisms can fail. The adaptive immune system receptors can recognize
nonmicrobial structures. Although most innate immune system receptors recognize
microbial structures, some Toll-like receptors and activating receptors of natural killer
cells do recognize nonmicrobial self proteins expressed by stressed, damaged, or infected
cells. Memory is a unique property of the adaptive and not the innate immune system.
4. Toll-like receptors (TLRs) are a family of homologous receptors expressed on many
cell types and are involved in innate immune responses. Ten different mammalian TLRs
have been identified, and several ligands for many of these receptors are known. Which
of the following is a TLR ligand?
A. Single-stranded RNA
B. Transfer RNA
C. Double-stranded DNA
D. Unmethylated CpG DNA
More than 10 mammalian Toll-like receptors (TLRs) have been identified, and each
appears to recognize a different set of structures that are found in pathogenic microbes
but not in mammalian cells. Such structures are called pathogen-associated molecular
patterns (PAMPs). Unmethylated cytosine guanosine (CpG) motifs are typical of bacterial
and protozoan DNA, but not mammalian DNA, and are therefore PAMPs. TLR9 binds
CpG DNA. Transfer RNA, single-stranded RNA, double-stranded DNA, and
heterochromatin are all normal components of mammalian cells and are not recognized
by TLRs. Double-stranded RNA is produced by some viruses but not by mammalian cells
and is recognized by TLR3.
5. A 67-year-old homeless man is brought to the emergency department after being
found behind a neighborhood bar in freezing weather. On arrival, he has a shaking chill,
fever, and cough productive of blood-tinged sputum. A chest radiograph shows lobar
consolidations consistent with bacterial pneumonia. Blood cultures are positive for
Streptococcus pneumoniae. Which of the following molecular patterns recognized by
Toll-like receptors expressed on the surface of this patient’s phagocytes is important for
activating his innate immune system against this gram-positive bacterial infection?
B. Double-stranded RNA
C. Lipopolysaccharide (LPS)
E. Phosphatidylinositol dimannoside
Gram-positive bacteria contain cell walls rich in peptidoglycan. When shed by bacteria
such as Streptococcus pneumoniae, peptidoglycan serves as a ligand that binds Toll-like
receptor 2 (TLR2), stimulating an innate immune response. The other choices listed are
also ligands that stimulate TLRs, but they are not present in gram-positive bacteria.
Double-stranded RNA is found in replicating viruses, lipopolysaccharide (LPS) is a
component of the outer cell wall of gram-negative bacteria, and both lipoarabinomannan
and phosphatidylinositol dimannoside are present in mycobacteria.
6. The signaling pathways triggered by Toll-like receptors typically result in activation
of which of the following pairs of transcription factors?
A. NFAT and T-bet
B. AP-1 and GATA-3
C. Fos and STAT-6
D. NFB and AP-1
E. Lck and Jun
The predominant signaling pathway used by Toll-like receptors (TLRs) results in the
activation of the NF-B transcription factor. Ligand binding to the TLR at the cell surface
leads to recruitment of several cytoplasmic signaling molecules through specific domaindomain interactions, resulting in degradation of IB and subsequent activation of NFB.
In some cell types, certain TLRs also engage other signaling pathways, such as the MAP
kinase cascade, leading to activation of the AP-1 transcription factor. T-bet and GATA-3
are transcriptional regulators involved in helper T cell differentiation. Fos is a component
of AP-1, and STAT-6 is a transcription factor activated by IL-4 binding to cells. Lck is
not a transcription factor, but rather a tyrosine protein kinase involved in antigen-receptor
signaling in T cells.
7. Toll-like receptors and other receptors are potent activators of various components of
the innate immune system. All of the following proteins are expressed in response to
signaling by these receptors EXCEPT:
C. Tumor necrosis factor
D. Inducible nitric oxide synthase (iNOS)
CD28, the activating receptor for B7-1 and B7-2 costimulatory molecules, is
constitutively expressed on the surface of many T cells and is not induced by Toll-like
receptor (TLR) signaling. TLR signaling does induce expression of B7-1 and B7-2 on
antigen-presenting cells. Other genes expressed in response to TLR signaling encode
proteins important in many different components of innate immune responses. These
include inflammatory cytokines such as tumor necrosis factor- (TNF-), interleukin-1
(IL-1), and IL-12; endothelial adhesion molecules such as E-selectin; and proteins
involved in microbial killing mechanisms, including inducible nitric oxide synthase
(iNOS). The specific genes expressed depend on the cell type of the responding cell.
8. Which of the following is a receptor on macrophages that is specific for a structure
produced by bacteria but not by mammalian cells?
A. CD36 (scavenger receptor)
B. Fc receptor
C. Complement receptor
D. Mannose receptor
The macrophage mannose receptor binds to terminal mannose and fucose residues on
bacterial glycoproteins and glycolipids. Mammalian cells do not typically contain these
residues. CD36 binds many different ligands, including microbial and self molecules. Fc
receptors, complement receptors, and ICAM-1 are receptors for mammalian complement
fragments, Ig, and LFA-1, respectively.
9. Which one of the following comparisons between neutrophils and macrophages is
A. Neutrophils that enter inflammatory sites can survive for days, but macrophages
are very short lived and only survive for hours.
B. Both neutrophils and macrophages are phagocytic and can kill internalized
C. Neutrophils proliferate at inflammatory sites, but macrophages are terminally
differentiated and cannot proliferate.
D. Neutrophils, but not macrophages, express the high-affinity FcRI receptor, which
recognizes specific opsonins bound to microbes and facilitates phagocytosis.
E. Both neutrophils and macrophages contain abundant cytoplasmic granules
containing lysozyme, collagenase, and elastase.
Neutrophils and macrophages can both actively phagocytose and kill microbes, and both
express opsonin receptors, such as FcRI or complement receptors that enhance
phagocytosis. Neutrophils are short lived, whereas macrophages can survive for days or
weeks. Macrophages are not terminally differentiated and can undergo cell division at
inflammatory sites, but neutrophils cannot. Only neutrophils have cytoplasmic granules
filled with enzymes, including lysozyme, collagenase, and elastases; these are called
10. A 43-year-old man with a history of kidney transplantation is on immunosuppressive
drugs. He presents to the emergency department 84 days after transplantation with a slight
fever, accompanied by violent shaking chills, rapid heart rate, and dangerously low blood
pressure. Blood cultures are positive for gram-negative bacteria, including Klebsiella and
Pseudomonas. Although the patient was initially alert and responsive to fluids and
antibiotic therapy, his condition rapidly deteriorates into disseminated intravascular
coagulation (DIC), hypoglycemia, and cardiovascular failure. Which of the following is
an essential mediator of this patient’s condition?
A. Transforming growth factor-
B. Tumor necrosis factor-
C. Interleukin (IL)-2
This patient is suffering from septic shock, characterized by the clinical triad of
disseminated intravascular coagulation (DIC), hypoglycemia, and cardiovascular failure.
This condition is most often initiated by endotoxin, also known as lipopolysaccharide
(LPS), a component of the outer cell walls of gram-negative bacteria. LPS is a potent
stimulus for tumor necrosis factor (TNF)- secretion by mononuclear phagocytes and
other cell types. Most of the biologic effects of LPS are mediated through TNF-.
Transforming growth factor- (TGF-) and interleukin (IL)-10 are anti-inflammatory
cytokines, IL-2 is a T cell growth factor, and IL-3 is a hematopoietic cytokine. These
cytokines are not mediators of septic shock.
11. Macrophages and neutrophils express several enzymes that are involved in
biochemical mechanisms that kill ingested microbes. Which of the following is NOT an
enzyme expressed by these cells?
A. Inducible nitric oxide synthase (iNOS)
B. Granzyme B
C. Phagocyte oxidase
Granzyme B, a proteolytic enzyme component of cytolytic T lymphocyte (CTL) and
natural killer (NK) cell granules, is involved in initiating caspase-dependent CTL killing
of target cells. Granzyme B is not involved in phagocyte killing of ingested microbes.
Inducible nitric oxide synthase (iNOS) generates NO in macrophages, and NO is toxic to
microbes. Phagocyte oxidase and myeloperoxidase are involved in generating free radical
species that kill ingested microbes in phagocytes. Lysozyme is a proteolytic enzyme in
neutrophil granules that contributes to microbial killing.
12. All of the following molecules are opsonins that facilitate efficient phagocytosis of
microbes by neutrophils and macrophages EXCEPT:
C. C-reactive protein
E. Mannose-binding lectin
C5a is a peptide released after cleavage of C5 protein during the complement cascade. It
stimulates the influx of neutrophils to the site of infection, thus acting as a
chemoattractant, not as an opsonin. C3b (covalently bound to microbes on which
complement activation has taken place) and IgG bound to antigen, are particularly potent
opsonins, because phagocytes have receptors for both C3b and the Fc region of IgG. Creactive protein and mannose-binding lectin also can coat microbes and be recognized by
phagocyte receptors; thus they serve as opsonins.
13. A 3-year-old boy, who is small for his age, has a history of pyogenic (pus-producing)
infections and cutaneous skin abscesses. Physical examination is remarkable for high
fever, enlarged liver and spleen, and swollen cervical lymph nodes. A culture from an
abscess on his arm reveals Staphylococcus aureus, a gram-positive bacteria that is also
catalase-positive. Immunoglobulin and complement levels are normal. Results of the
nitroblue tetrazolium test are consistent with a diagnosis of chronic granulomatous
disease (CGD). The boy’s immunodeficiency involves impaired generation of which of
B. C-reactive protein
C. Mannose-binding lectin
D. Reactive oxygen intermediates
E. Membrane attack complex
Chronic granulomatous disease (CGD) is a rare, inherited immunodeficiency disease
associated with a defective intracellular respiratory burst in phagocytes. It consists of a
group of heterogeneous disorders of oxidative metabolism in which the pathways
required for generation of toxic reactive oxygen species (ROIs) are impaired. In patients
with CGD, phagocytosis occurs normally, but the engulfed microbes are not killed and
they multiply within the cell. In this way, patients are susceptible to recurrent infections
with organisms such as Staphylococcus, which are of low virulence in normal hosts.
14. A 4-year-old-girl sees her physician because of a severe necrotizing, oropharyngeal
herpes simplex viral (HSV) infection. She has a past medical history of cytomegalovirus
(CMV) pneumonitis and cutaneous HSV infection. Phenotypic analysis of her blood cells
shows an absence of CD56+ and CD16+ cells. There are normal numbers of CD4+ and
CD8+ cells in the blood, and serum antibody titers are normal. The patient’s CD8+ T
cells were able to kill virally infected target cells in vitro. Which of the following is NOT
characteristic of this girl’s immunodeficiency disease?
A. Lack of cells whose activation is normally inhibited by self class I major
histocompatibility complex (MHC)
B. Impaired granzyme B–dependent killing of virally infected target cells
C. Lack of cells that are activated by IL-15
D. Impaired interferon (IFN)-production during early phases of viral infection
E. Failure to form viral peptide-class I MHC complexes
The presence of normal numbers of CD8+ T cells and the ability of these cells to kill
virally infected target cells indicates that the class I major histocompatibility complex
(MHC) pathway of viral peptide antigen presentation is intact. The patient’s
immunodeficiency is due to a lack of natural killer (NK) cells. NK cells express CD56
and/or CD16. NK cells are activated by interleukin-15 (IL-15) and IL-12, are normally
inhibited by recognizing class I MHC on other cells, kill target cells with altered class I
MHC expression through a granzyme B–dependent mechanisms (similar to cytolytic T
lymphocyte killing), and produce interferon- as part of the early innate response to viral
15. Which one of the following statements about inhibitory receptors on natural killer
(NK) cells is true?
A. Inhibitory receptors on NK cells express ITAM motifs in their cytoplasmic tails.
B. Some inhibitory receptors on NK cells recognize HLA-A or HLA-C.
C. Some inhibitory receptors on NK cells are members of the integrin family.
D. Some inhibitory receptors on NK cells are members of the Toll-like receptor
E. Inhibitory receptors on NK cells are not expressed on the same NK cells that
express activating receptors.
Natural killing (NK) inhibitory receptors recognize class I MHC molecules that are
normally and constitutively expressed, including various alleles of HLA-A and HLA-C.
The cytoplasmic tails of NK inhibitory receptors contain immunoreceptor tyrosine-based
inhibitory motifs (ITIMs), but not immunoreceptor tyrosine-based activation motifs
(ITAMs). Some inhibitory receptors on NK cells are members of the Ig superfamily, but
not the integrin or TLR families. NK cells usually express both activating and inhibitory
receptors, and activation is regulated by a balance between signals generated from both
types of receptors. The inhibitory receptors on NK cells bind to self class I MHC
molecules, which are expressed on most normal cells. When activating and inhibitory
receptors are simultaneously engaged, the inhibitory receptor signals dominate and the
NK cell is not activated.
16. Complement activation in the innate immune system can be initiated in the absence of
antibody. Which of the following molecular components of the complement system is
involved in initiation of antibody-independent complement activation?
C. Mannose binding lectin
E. Mannose receptor
Mannose-binding lectin (MBL) is a soluble serum component that is structurally similar
to C1 of the classical complement pathway. MBL binds to mannan residues on microbial
surfaces and triggers proteolytic cleavage and activation of downstream components of
the complement system. C9 is not involved in initiation of complement activation but is
part of the common final membrane attack complex (MAC) pathway. CR2 is a cell
surface receptor for complement fragments. A mannose receptor is a cell surface receptor
on phagocytes that binds mannan residues and promotes phagocytosis of microbes.
17. Which of the following is an example of how the innate immune response stimulates
or modifies adaptive immunity?
A. Tumor necrosis factor (TNF) secreted by helper T cells enhances adhesion
molecules on endothelial cells and promotes recruitment of inflammatory cells.
B. Interferon (IFN)- produced by T helper cells is a potent activator of macrophages,
allowing killing of phagocytosed microbes.
C. B7-1 expression on antigen-presenting cells is up-regulated in response to
signaling through Toll-like receptors, thus enabling costimulation of T cells.
D. Infected cells coated by IgG3 are recognized by Fc receptors on natural killer
cells, allowing efficient killing of the infected cells.
E. Double-stranded RNA of replicating viruses potently stimulates IFN- expression
by fibroblasts, inducing an “antiviral state” in neighboring, uninfected cells.
Innate immune responses are important stimulators of adaptive immunity. Increased
expression of B7-1 and B7-2 on antigen-presenting cells after microbial activation of
Toll-like receptors (innate immunity) is critical in providing costimulatory signals for T
cell activation (adaptive immunity) via binding to CD28 receptors on T cells. T helper
cell–mediated endothelial or macrophage activation is an example of adaptive immunity
using the effector mechanisms of innate immunity. Neither IgG3 opsonization facilitating
natural killer cytolytic activity nor double-stranded RNA stimulating interferon-
secretion involve innate immunity enhancing adaptive immunity.