8. When a B-cell undergoes immunoglobulin class switching
a. the variable region of the light chain changes, its constant region stays the same
b. the variable region of the light chain stays the same, its constant region changes
c. the variable region of the heavy chain stays the same, its constant region changes
d. the variable region of the heavy chain changes, its constant region stays the same

During immunoglobulin class switching, activated B-cells change antibody isotypes from IgM to IgG, IgA, or IgE while preserving antigen specificity. The correct answer is option c: the variable region of the heavy chain stays the same, its constant region changes.​

Option Analysis

a. Variable region of light chain changes, constant stays same
Light chain variable (V_L) and constant (C_L) regions remain unchanged during class switching, as recombination targets heavy chain loci only. This process preserves the antigen-binding site across both chains.​

b. Variable region of light chain stays same, constant changes
Light chain constant regions (κ or λ) do not switch; only heavy chain constant regions (C_μ to C_γ, etc.) are replaced via DNA deletion. Light chains maintain fixed isotypes.​

c. Variable region of heavy chain stays same, constant changes
Class switch recombination (CSR) deletes intervening DNA between switch (S) regions, linking the unchanged VDJ-recombined heavy chain variable region (V_H) to a new downstream constant region. Antigen specificity persists.​

d. Variable region of heavy chain changes, constant stays same
Variable regions form during V(D)J recombination in immature B-cells and remain fixed post-activation; CSR exclusively alters constant regions.​

Mechanism Overview

Class switching occurs in mature B-cells after antigen activation and T-cell help, mediated by activation-induced cytidine deaminase (AID). AID deaminates cytosines in S regions upstream of constant genes (except C_δ), creating double-strand breaks repaired by non-homologous end joining (NHEJ). This deletional recombination swaps C_μ (IgM) for other isotypes, enabling diverse effector functions like opsonization (IgG) or mucosal immunity (IgA).​

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