1 BASIC PRINCIPLES OF IMMUNOLOGY

Question 1

(1) From the following, identify a specific component of the adaptive immune system that is formed in response to antigenic stimulation:

A. Lysozyme
B. Complement
C. Commensal organisms
D. Immunoglobulin (Ig)

Answer 1

D. Immunoglobulin (Ig)

Ig is a specific part of the adaptive immune system and is formed only in response to a specific antigenic stimulation. Complement, lysozyme, and commensal organisms all act nonspecifically as a part of the adaptive immune system. These three components do not require any type of specific antigenic stimulation.

Question 2

(2) Which two organs are considered the primary lymphoid organs in which immunocompetent cells originate and mature?

A. Thyroid and Peyer patches
B. Thymus and bone marrow
C. Spleen and mucosal-associated lymphoid tissue (MALT)
D. Lymph nodes and thoracic duct

Answer 2

B. Thymus and bone marrow

Bone marrow and the thymus are considered primary lymphoid organs because immunocompetent cells either originate from them or mature in them. Some immunocompetent cells mature or reside in bone marrow (the source of all hematopoietic cells) until transported to the thymus, spleen, or Peyer patches, where they process antigen or manufacture antibody. T lymphocytes, after originating in bone marrow, travel to the thymus to mature and differentiate.

Question 3

(3) What type of B cells is formed after antigen stimulation?

A. Plasma cells and memory B cells
B. Mature B cells
C. Antigen-dependent B cells
D. Receptor-activated B cells

Answer 3

A. Plasma cells and memory B cells

Mature B cells exhibit surface Ig that may cross-link a foreign antigen, thus forming the activated B cell and leading to capping and internalization of antigen. The activated B cell gives rise to plasma cells that produce and secrete Igs and memory cells that reside in lymphoid organs.

Question 4

(4) T cells travel from bone marrow to the thymus for maturation. What is the correct order of the maturation sequence for T cells in the thymus?

A. Bone marrow to the cortex; after thymic education, released back to peripheral circulation
B. Maturation and selection occur in the cortex; migration to the medulla; release of mature T cells to secondary lymphoid organs
C. Storage in either the cortex or medulla; release of T cells into the peripheral circulation
D. Activation and selection occur in the medulla; mature T cells are stored in the cortex until activated by antigen

Answer 4

B. Maturation and selection occur in the cortex; migration to the medulla; release of mature T cells to secondary lymphoid organs

Immature T cells travel from bone marrow to the thymus to mature into functional T cells. Once in the thymus, T cells undergo a selection and maturation sequence that begins in the cortex and moves to the medulla of the thymus. Thymic factors, such as thymosin and thymopoietin, and cells within the thymus, such as macrophages and dendritic cells, assist in this sequence. After completion of the maturation cycle, T cells are released to secondary lymphoid organs to await antigen recognition and activation.

Question 5

(5) Which cluster of differentiation (CD) marker is the most specific identifying marker for mature T cells?

A. CD1
B. CD2
C. CD3
D. CD4 or CD8

Answer 5

C. CD3

The CD3 marker appears during the early stages of T-cell development and can be used to differentiate T cells from other lymphocytes.

Question 6

(6) Which markers are found on mature, peripheral helper T cells?

A. CD1, CD2, CD4
B. CD2, CD3, CD8
C. CD1, CD3, CD4
D. CD2, CD3, CD4

Answer 6

D. CD2, CD3, CD4

Mature, peripheral helper T cells have the CD2, CD3 (mature T cell), and CD4 (helper) markers.

Question 7

(7) Which T cells express the CD8 marker and act specifically to kill tumors or virally infected cells?

A. Helper T cells
B. Suppressor T cells
C. Cytotoxic T cells (TC cells)
D. Regulator T cells

Answer 7

C. Cytotoxic T cells (TC cells)

TC cells recognize antigen in association with major histocompatibility complex (MHC) class I complexes and act against target cells that express foreign antigens. These include viral antigens and the HLAs that are the target of graft rejection.

Question 8

(8) How are TC cells and natural killer (NK) cells similar?

A. Require antibody to be present
B. Effective against virally infected cells
C. Recognize antigen in association with human leukocyte antigen (HLA) class II markers
D. Do not bind to infected cells

Answer 8

B. Effective against virally infected cells

Both TC and NK cells are effective against virally infected cells, and neither requires antibody to be present to bind to infected cells. NK cells do not exhibit MHC class restriction, whereas activation of TC cells requires the presence of MHC class I molecules in association with the viral antigen.

Question 9

(9) What is the name of the process by which phagocytic cells are attracted to a substance, such as a bacterial peptide?

A. Diapedesis
B. Degranulation
C. Chemotaxis
D. Phagotaxis

Answer 9

C. Chemotaxis

Chemotaxis is the process by which phagocytic cells are attracted toward an area where they detect a disturbance in the normal functions of body tissues. Products from bacteria and viruses, complement components, coagulation proteins, and cytokines
from other immune cells may all act as chemotactic factors.

Question 10

(10) All of the following are immunologic functions of complement except:

A. Induction of an antiviral state
B. Opsonization
C. Chemotaxis
D. Anaphylatoxin formation

Answer 10

A. Induction of an antiviral state

Complement components are serum proteins that function in opsonization, chemotaxis, and anaphylatoxin formation but do not induce an antiviral state in target cells. This function is performed by interferons.

Question 11

(11) Which complement component is found in both the classic and alternative pathways?

A. C1
B. C4
C. Factor D
D. C3

Answer 11

D. C3

C3 is found in both the classic and alternative (alternate) pathways of the complement system. In the classic pathway, C3b forms a complex on the cell with C4b2a that enzymatically cleaves C5. In the alternative pathway, C3b binds to an activator on the cell surface. It forms a complex with factor B called C3bBb, which, like C4b2a3b, can split C5.

Question 12

(12) Which Ig(s) help(s) initiate the classic complement pathway?

A. IgA and IgD
B. IgM only
C. IgG and IgM
D. IgG only

Answer 12

C. IgG and IgM

Both IgG and IgM are the Igs that help to initiate the activation of the classic complement pathway. IgM is, however, a more potent complement activator.

Question 13

(13) How is complement activity destroyed in vitro?

A. Heating serum at 56°C for 30 minutes
B. Keeping serum at room temperature of 22°C for 1 hour
C. Heating serum at 37°C for 45 minutes
D. Freezing serum at 0°C for 24 hours

Answer 13

A. Heating serum at 56°C for 30 minutes

Complement activity in serum, in vitro, is destroyed by heating serum at 56°C for 30 minutes. In test procedures where complement may interfere with the test system, it may be necessary to destroy complement activity in the test sample by heat inactivation.

Question 14

(14) What is the purpose of C3a, C4a, and C5a, the split products of the complement cascade?

A. To bind with specific membrane receptors of lymphocytes and cause release of cytotoxic substances
B. To cause increased vascular permeability, contraction of smooth muscle, and release of histamine from basophils
C. To bind with membrane receptors of macrophages to facilitate phagocytosis and the removal of debris and foreign substances
D. To regulate and degrade membrane cofactor protein after activation by C3 convertase

Answer 14

B. To cause increased vascular permeability, contraction of smooth muscle, and release of histamine from basophils

C3a, C4a, and C5a are split products of the complement cascade that participate in various biological functions, such as vasodilation and smooth muscle contraction. These small peptides act as anaphylatoxins, for example, effector molecules that participate in the inflammatory response to assist in the destruction and clearance of foreign antigens.

Question 15

(15) Which region of the Ig molecule can bind antigen?

A. Fragment antigen binding (Fab)
B. Fragment crystallizable (Fc)
C. Constant light (CL)
D. Constant heavy (CH)

Answer 15

A. Fragment antigen binding (Fab)

Fab is the region of the Ig molecule that can bind antigen. Two Fab fragments are formed from hydrolysis of the Ig molecule by papain. Each consists of a light chain and the VH and CH1 regions of the heavy chain. The variable regions of the light and heavy chains interact, forming a specific antigen-combining site.

Question 16

(16) Which region determines whether an Ig molecule can fix complement?

A. Variable heavy (VH)
B. Constant heavy (CH)
C. Variable light (VL)
D. Constant light (CL)

Answer 16

B. Constant heavy (CH)

The composition and structure of the constant region of the heavy chain determine whether that Ig will fix complement. Fc is formed by partial Ig digestion with papain and includes the CH2 and CH3 domains of both heavy chains. The complement component C1q molecule will bind to the CH2 region of an IgG or IgM molecule.

Question 17

(17) Which Ig class(es) has (have) a J-chain?

A. IgM
B. IgE and IgD
C. IgM and surface IgA (sIgA)
D. IgG3 and IgA

Answer 17

C. IgM and surface IgA (sIgA)

Both IgM and sIgA have a J-chain joining individual molecules together; the J-chain in IgM joins five molecules and the J-chain in sIgA joins two molecules.

Question 18

(18) Which Ig appears first in the primary immune response?

A. IgG
B. IgM
C. IgA
D. IgE

Answer 18

B. IgM

The first antibody to appear in the primary immune response to an antigen is IgM. The titer of antiviral IgM (e.g., IgM antibody to cytomegalovirus [anti CMV]) is more specific for acute or active viral infection than IgG and may be measured to help differentiate active infection from prior infection.

Question 19

(19) Which immunoglobulin appears in highest titer in the secondary response?

A. IgG
B. IgM
C. IgA
D. IgE

Answer 19

A. IgG

A high titer of IgG characterizes the secondary immune response. Consequently, IgG antibodies comprise about 80% of the total Ig concentration in normal serum.

Question 20

(20) Which Ig can cross the placenta?

A. IgG
B. IgM
C. IgA
D. IgE

Answer 20

A. IgG

IgG is the only Ig class that can cross the placenta. All subclasses of IgG can cross the placenta, but IgG2 crosses more slowly. This process requires recognition of the Fc region of the IgG by placental cells. These cells take up the IgG from maternal blood and secrete it into fetal blood, providing humoral immunity to the neonate for the first few months after delivery.

Question 21

(21) Which Ig cross-links mast cells to release histamine?

A. IgG
B. IgM
C. IgA
D. IgE

Answer 21

D. IgE

IgE is the Ig that cross-links with basophils and mast cells. IgE causes the release of such immune response modifiers as histamine and mediates an allergic immune response.

Question 22

(22) All of the following are functions of Igs except:

A. Neutralizing toxic substances
B. Facilitating phagocytosis through opsonization
C. Interacting with TC cells to lyse viruses
D. Combining with complement to destroy cellular antigens

Answer 22

C. Interacting with TC cells to lyse viruses

Tc cells lyse virally infected cells directly, without requirement for specific antibody. The TC cell is activated by viral antigen that is associated with MHC class I molecules on the surface of the infected cell. The activated TC cell secretes several toxins, such as tumor necrosis factor, which destroy the infected cell and virions.

Question 23

(23) Which of the following cell surface molecules is classified as an MHC class II antigen?

A. HLA-A
B. HLA-B
C. HLA-C
D. HLA-DR

Answer 23

D. HLA-DR

The MHC region is located on the short arm of chromosome 6 and codes for antigens expressed on the surface of leukocytes and tissues. The MHC region genes control immune recognition; their products include the antigens that determine transplant rejection. HLA-DR antigens are expressed on B cells. HLA-DR2, -DR3, -DR4, and -DR5 antigens show linkage with a wide range of autoimmune diseases.

Question 24

(24) Which MHC class of molecule is necessary for antigen recognition by CD4-positive T cells?

A. Class I
B. Class II
C. Class III
D. No MHC molecule is necessary for antigen recognition

Answer 24

B. Class II

Helper T lymphocytes (CD4-positive T cells) recognize antigens only in the context of a class II molecule. Because class II antigens are expressed on macrophages, monocytes, and B cells, the helper-T-cell response is mediated by interaction with processed antigen on the surface of these cells.

Question 25

(25) Which of the following are products of HLA class III genes?

A. T-cell immune receptors
B. HLA-D antigens on immune cells
C. Complement proteins C2, C4, and factor B
D. Ig VL regions

Answer 25

C. Complement proteins C2, C4, and factor B

Complement components C2 and C4 of the classic pathway and factor B of the alternative pathway are class III molecules. HLA-A, HLA-B, and HLA-C antigens are classified as class I antigens, and HLA-D, HLA-DR, HLA-DQ, and HLA-DP antigens are classified as class II antigens.

Question 26

(26) What molecule on the surface of most T cells recognizes antigen?

A. IgT, a four-chain molecule that includes the tau heavy chain
B. MHC protein, a two-chain molecule encoded by the HLA region
C. CD3, consisting of six different chains
D. T-cell receptor (TcR), consisting of two chains: α chain and β-chain

Answer 26

D. T-cell receptor (TcR), consisting of two chains: α chain and β-chain

T cells have a membrane bound receptor (TcR) that is antigen specific. This twochain molecule consists of a single α-chain, similar to an Ig light chain, and a single β-chain, similar to an Ig heavy chain. Some T cells may express a γ-δ receptor instead of the α-β molecule. There is no τ heavy chain. MHC and CD3 molecules are present on T cells, but they are not the molecules that give antigen specificity to the cell.

Question 27

(27) TcR is similar to Ig molecules in that it:

A. Remains bound to the cell surface and is never secreted
B. Contains V and C regions on each of its chains
C. Binds complement
D. Can cross the placenta and provide protection to a fetus

Answer 27

B. Contains V and C regions on each of its chains

The antigen binding regions of both the α- and β chains of the TcR are encoded by V genes that undergo rearrangement similar to that observed in Ig genes. The α-chain gene consists of V and J segments, similar to an Ig light chain. The β-chain consists of V, D, and J segments, similar to an Ig heavy chain. The α- and β-chains each have a single C-region gene encoding the constant region of the molecule. Although answer A is true for TcRs, it is not true for Igs that can be cell bound or secreted. Answers C and D are true for certain Ig heavy-chain isotypes but are not true for the TcR.

Question 28

(28) Toll-like receptors (TLRs) are found on which cells?

A. T cells
B. Dendritic cells
C. B cells
D. Large granular lymphocytes

Answer 28

B. Dendritic cells

TLRs are the primary antigen recognition protein of the innate immune system. They are found on antigen-presenting cells, such as dendritic cells and macrophages. Eleven TLRs have been described. TLRs recognize certain structural motifs common to infecting organisms. TLR 4, for example, recognizes bacterial lipopolysaccharide (LPS). The name TLR comes from its similarity to the Toll protein in Drosophila.

Question 29

(29) Macrophages produce which of the following proteins during antigen processing?

A. IL-1 and IL-6
B. γ-Interferon
C. IL-4, IL-5, and IL-10
D. Complement components C1 and C3

Answer 29

A. IL-1 and IL-6

Interleukin-1 (IL-1) and IL-6 are proinflammatory macrophage-produced cytokines. In addition to their inflammatory properties, they activate T helper cells during antigen presentation. γ-Interferon, IL-4, IL-5, and IL-10 are all produced by T cells. Complement components are produced by a variety of cells but are not part of the macrophage antigen presentation process.

Question 30

(30) A superantigen, such as toxic shock syndrome toxin-1 (TSST-1), bypasses the normal antigen processing stage by binding to and cross-linking:

A. A portion of an Ig molecule and complement component C1
B. TLRs and an MHC class 1 molecule
C. A portion of an Ig and a portion of a TcR
D. A portion of a TcR and an MHC class II molecule

Answer 30

D. A portion of a TcR and an MHC class II molecule

A superantigen binds to the V β-portion of the TcR and an MHC class II molecule. This binding can activate T cells without the involvement of an antigen-presenting cell. In some individuals, a single V β-protein that recognizes TSST-1 is expressed on up to 10% to 20% of T cells. The simultaneous activation of this amount of T cells causes a heavy cytokine release, resulting in the vascular collapse and pathology of toxic shock syndrome

Question 31

(31) T-regulator cells, responsible for controlling autoimmune antibody production, express which of the following phenotypes?

A. CD3, CD4, CD8
B. CD3, CD8, CD25
C. CD3, CD4, CD25
D. CD8, CD25, CD56

Answer 31

C. CD3, CD4, CD25

T-regulator cells are believed to be the primary immune suppressor cells and express CD3, CD4, and CD25. CD25 is the IL-2 receptor. CD25 may be expressed by activated T cells, but is constitutively expressed by the T-regulator cells. CD25 expression on T-regulator cells occurs in the thymus and is regulated by the FOXP3 protein.