MCQs in Immunology (21-30): Antibodies-I (Answers with Explanations)

21. All of the following are true concerning IgM antibodies EXCEPT which one?

a. They are glycoproteins

b. They fix complement

c. They mediate allergic reactions

d. They occur on the surface of lymphocytes

e. They predominate in the primary response to antigen

22. All of the following are true concerning IgE molecules, EXCEPT which one?

a. They are involved in mediating anti-parasitic immune responses.

b. They are the least abundant immunoglobulin in the serum.

c. They are the principal immunoglobulin class involved in allergic reactions.

d. They can affect the release of histamine and other chemical mediators.

e. They will cross the placenta and fix the complement.

23. Which immunoglobulins are generally present in plasma at the highest concentration?

a. IgA

b. IgD

c. IgE

d. IgG

e. IgM

24. All of the following are true about antibodies, EXCEPT which one?

a. They are glycoproteins.

b. They are molecules with a single, defined amino acid sequence.

c. They fix complement.

d. They occur on the surface of B-lymphocytes.

e. They predominate the primary immune response to antigen.

25. The major immunoglobulin family to which a particular immunoglobulin belongs can be determined by sequential analysis of the 110 amino acids beginning from the:

a. Amino terminus of the heavy chain.

b. Amino terminus of the light chain.

c. Carboxy terminus of the heavy chain.

d. Carboxy terminus of the light chain.

e. None of the above

26. The immunoglobulin Joining chain (J-chain) is:

a. Only produced by T-cells

b. Only produced by neutrophils

c. Associated with only multimeric forms of IgM and IgA

d. Associated with IgE for histamine release

e. Only produced by mast cells

27. All of the following are true EXCEPT:

a. An antigenic determinant is a paratope

b. An epitope is a small portion of a macromolecule

c. An IgG antibody is bivalent

d. The class of immunoglobulin is determined by its heavy chain

e. The variable region domains contain the antigen recognition site

28. Which immunoglobulin is the principal one found in secretions such as milk?

a. IgA

b. IgD

c. IgE

d. IgG

e. IgM

29. Which of the following statements best characterizes an antibody?

a. An antibody contains high molecular weight RNA as its basic structure.

b. An antibody is composed of protein and cannot be distinguished from the albumin fraction of the serum proteins.

c. An antibody is composed of four identical protein subunits, which may be caused to dissociate by treatment with urea.

d. An antibody contains protein as its major chemical component, and its synthesis may be elicited by administering a foreign protein or polysaccharide.

e. An antibody contains mucopolysaccharides as its major chemical component, and the synthesis of these may be elicited by administering a foreign protein or polysaccharide.

30. The class of an immunoglobulin:

a. Is determined by Class I and Class II major histocompatibility complex proteins

b. Is determined by the carbohydrate attached to the light chain

c. Is determined by the antigen

d. Is determined by the heavy chain type

e. Is determined by the J-chain

Practice more MCQs:MCQ in Immunology (31-40): Complement System

Immunology MCQs: Clean Text Answer Key & Explanations

**21. Correct Answer: c (They mediate allergic reactions)**

• Explanation: This is the incorrect statement regarding IgM, making it the correct exception. IgM is a pentameric or monomeric glycoprotein that is excellent at fixing complement and dominates the primary immune response. Allergic reactions (Type I hypersensitivity) are mediated strictly by IgE, not IgM.

**22. Correct Answer: e (They will cross the placenta and fix the complement)**

• Explanation: This statement is false for IgE, which makes it the correct exception. IgG is the only immunoglobulin class capable of crossing the human placenta to provide passive immunity to the fetus, and IgE does not classicaly fix complement. IgE is, however, the least abundant serum antibody, drives anti-parasitic immunity, and triggers histamine release from mast cells.

**23. Correct Answer: d (IgG)**

• Explanation: IgG is the predominant immunoglobulin class in human serum and interstitial spaces, accounting for roughly 75% to 80% of the total pool of circulating antibodies. This high concentration ensures robust systemic protection against bacterial and viral pathogens.

**24. Correct Answer: b (They are molecules with a single, defined amino acid sequence)**

• Explanation: This is the false statement and the correct exception. Antibodies are fundamentally heterogeneous. Due to V(D)J genetic recombination, somatic hypermutation, and class switching, an individual produces an immense pool of polyclonal antibodies with highly diverse amino acid sequences in their variable domains, rather than a single uniform sequence.

**25. Correct Answer: c (Carboxy terminus of the heavy chain)**

• Explanation: The constant domains (CH) located at the carboxy-terminal (C-terminus) end of the heavy chains carry the distinct, constant amino acid sequences that define the immunoglobulin class or isotype (e.g., mu for IgM, gamma for IgG, alpha for IgA). The amino terminus (N-terminus), by contrast, contains the variable antigen-binding site.

**26. Correct Answer: c (Associated with only multimeric forms of IgM and IgA)**

• Explanation: The Joining (J) chain is a small glycoprotein synthesized by plasma cells. Its specific physiological function is to structurally link and stabilize polymeric immunoglobulins—specifically forming dimers of secretory IgA and pentamers of IgM. It is not found in monomeric antibodies like IgG or IgE.

**27. Correct Answer: a (An antigenic determinant is a paratope)**

• Explanation: This statement is incorrect, making it the right exception. An antigenic determinant is an epitope—the specific structural region on the surface of an antigen recognized by the immune system. A paratope is the matching antigen-binding site found on the antibody molecule itself.

**28. Correct Answer: a (IgA)**

• Explanation: Secretory IgA (sIgA) is the primary immunological goalkeeper of mucosal surfaces and external body secretions. It is highly abundant in colostrum, breast milk, saliva, tears, and mucous membranes, providing vital localized passive immunity to breastfed infants.

**29. Correct Answer: d (An antibody contains protein as its major chemical component, and its synthesis may be elicited by administering a foreign protein or polysaccharide.)**

• Explanation: This option accurately characterizes an antibody as an immunogenic glycoprotein induced by an antigen (like a foreign protein or polysaccharide). Options describing it as containing high molecular weight RNA or being purely a mucopolysaccharide are chemically incorrect; they are distinct from serum albumin.

**30. Correct Answer: d (Is determined by the heavy chain type)**

• Explanation: The structural class (isotype) of an antibody molecule (IgA, IgD, IgE, IgG, or IgM) is dictated solely by its heavy chain constant region. The heavy chains are designated by Greek letters (alpha, delta, epsilon, gamma, and mu, respectively), which determine the overall structural layout and biological effector functions.