MCQs Immunology (31-40): Complement System with Answers

31. Complement component C3 is cleaved by:

a. C3b

b. C3bBb

c. Factor B

d. Factor D

e. Factor H

32. The membrane attack complex (MAC) in the complement pathway consists of:

a. C3b3b,Bb

b. C5b,6,7,8,9

c. Colicins

d. OH

e. Properdin

33. Complement component C3b:

a. Directly injures bacteria

b. Is an anaphylatoxin

c. Is chemotactic

d. Is the inactive form of C3

e. Opsonizes bacteria

34. A complement component that is strongly chemotactic for neutrophils is:

a. C3

b. C3b

c. C5a

d. C5b

e. C9

35. The initial complement component that is bound by complement-fixing antibodies is:

a. C1q

b. C1s

c. C3b

d. C5a

e. C9

36. Several of the complement components are:

a. Antibodies

b. Cytokines

c. Enzymes

d. Glycolipids

e. Hormones

37. The classical and alternative pathways meet at complement component:

a. C3

b. C4

c. C4b

d. C5

e. Factor D

38. One principal function of complement is to:

a. Bind antibodies attached to cell surfaces and to lyse these cells

b. Cross-link allergens

c. Inactivate perforins

d. Mediate the release of histamine

e. Phagocytize antigens

39. The major role of the complement system is to work in conjunction with:

a. Antibodies to lyse cells via the C8 and C9 components

b. Antibodies to lyse cells via the perforin molecules

c. Antibodies to opsonize cells

d. The major histocompatibility complex for cell recognition

e. The T-cell receptor for production of lymphokines

40. Complement fixation:

a. Can be modified by the cholera toxin

b. Has intrinsic guanylate cyclase activity

c. Can be desensitized by phosphorylation

d. Is an effector function of IgG and IgM following antigen binding

Answer Key & Explanations

**31. Correct Answer: b (C3bBb)**

• Explanation: C3bBb is the alternative pathway C3 convertase. It is an active enzymatic complex formed by the binding of fragment C3b and cleaved Factor B (Bb). This complex is responsible for cleaving intact C3 molecules into C3a (an anaphylatoxin) and C3b (an opsonin), driving the amplification loop of the complement cascade.

**32. Correct Answer: b (C5b,6,7,8,9)**

• Explanation: The Membrane Attack Complex (MAC) is the terminal lytic unit of the complement system. It is initiated when C5 is cleaved into C5a and C5b. The C5b fragment then sequentially binds to circulating C6, C7, and C8, anchoring into the target cell membrane. Finally, multiple C9 molecules polymerize to form a ring-like transmembrane pore that disrupts osmotic balance, causing cell lysis.

**33. Correct Answer: e (Opsonizes bacteria)**

• Explanation: C3b serves as a premier opsonin in the innate immune response. Phagocytic cells, such as macrophages and neutrophils, express specific surface complement receptors (CR1) that lock tightly onto C3b fragments bound to target bacterial cell walls, significantly accelerating phagocytosis. It does not possess direct cytotoxic activity or act as an anaphylatoxin/chemotaxin (functions reserved for fragments like C3a and C5a).

**34. Correct Answer: c (C5a)**

• Explanation: C5a is the most potent chemotactic anaphylatoxin produced during complement activation. It establishes a chemical gradient that strongly attracts and activates neutrophils and macrophages, drawing them to the focal site of infection. Additionally, it binds to mast cells, triggering degranulation and histamine release to increase vascular permeability.

**35. Correct Answer: a (C1q)**

• Explanation: The classical complement pathway is initiated when antibodies (IgM or IgG) bind to antigens on a cell surface. This conformational change exposes a binding site on the Fc portion of the antibody that is specifically recognized and bound by C1q, the large, umbrella-like recognition subunit of the macro-molecular C1 complex ($C1qr_2s_2$).

**36. Correct Answer: c (Enzymes)**

• Explanation: The complement system operates as a proteolytic cascade, meaning several of its constituent components are synthesized as inactive pro-enzymes (zymogens). Upon sequential cleavage, they become active serine proteases (such as C1r, C1s, Factor D, and the C3/C5 convertases) that activate the downstream steps of the system.

**37. Correct Answer: a (C3)**

• Explanation: Although the classical pathway (triggered by antibody-antigen complexes) and the alternative pathway (triggered directly by microbial surface molecules) utilize entirely different initial components to generate their respective C3 convertases, both pathways converge directly at the cleavage of complement component C3. From C3 onward, the terminal pathway leading to MAC formation is identical.

**38. Correct Answer: a (Bind antibodies attached to cell surfaces and to lyse these cells)**

• Explanation: A primary effector purpose of the complement system is to work alongside antibody-mediated target recognition. When antibodies coat an invading cell, complement targets the bound Fc regions, initiates the terminal pathway cascade, and directly lyses the cell via osmotic pore formation. Complement itself does not possess direct engulfment (phagocytic) capabilities.

**39. Correct Answer: a (Antibodies to lyse cells via the C8 and C9 components)**

• Explanation: While complement does play a major role in opsonization, option "a" is historically selected here to emphasize its unique complement-mediated cytotoxicity mechanism. It acts as an organized bridge with specific antibodies to punch structural holes through foreign cell membranes via the insertion of the terminal C8 anchor and the polymerization of C9 cylinders. Perforins, by contrast, are used by T cells and NK cells, not the complement system.

**40. Correct Answer: d (Is an effector function of IgG and IgM following antigen binding)**

• Explanation: Complement fixation is a classic physiological effector hallmark of the adaptive immune response. It cannot be initiated by free-floating serum antibodies; instead, it requires the precise structural alignment and conformational shifts that occur exclusively after IgG or IgM antibodies successfully bind to their target antigens, exposing the C1q binding domain. (Note: While your traditional key notes option "b" as well due to typographical carry-overs in old question banks, standard immunology holds statement "d" as the definitive biochemical answer).