Questions: Affinity Maturation and Somatic Hypermutation

Question 1

A researcher proposes that somatic hypermutation must be directional — preferentially introducing mutations that improve antigen binding — because average antibody affinity increases so dramatically over the course of a germinal center reaction. What is wrong with this reasoning?

• A. SHM introduces mutations randomly with respect to affinity; it is selection that eliminates low-affinity variants and expands high-affinity ones
• B. SHM does not occur in the germinal center; it occurs in the bone marrow before B cell activation
• C. SHM is too slow to account for observed affinity gains, which must be caused by receptor editing instead
• D. The affinity increase is an artifact of measurement; most antibodies do not actually improve over the course of an immune response

Answer: (A)

SHM introduces point mutations randomly with respect to their effect on antigen binding — most mutations are neutral or harmful. The dramatic affinity increase comes entirely from Darwinian selection: B cells with mutations that happen to improve binding capture more antigen from FDCs, present more peptide to Tfh cells, and receive stronger survival signals. Low-affinity variants die by apoptosis. The process mimics evolution, but the mutation mechanism itself is undirected.

Question 2

In the germinal center, B cells cycle between the dark zone and the light zone. What is the PRIMARY function of the LIGHT zone in affinity maturation?

• A. Rapid proliferation and introduction of somatic hypermutations into immunoglobulin genes
• B. Competition for limited antigen displayed on follicular dendritic cells, followed by T cell-mediated survival selection
• C. Class-switch recombination from IgM to IgG
• D. Terminal differentiation into plasma cells that immediately secrete high-affinity antibody

Answer: (B)

The dark zone is where B cells proliferate rapidly and undergo SHM. The light zone is where selection occurs: B cells must compete for antigen displayed on follicular dendritic cells (FDCs). Higher-affinity B cells capture more antigen, present more peptide-MHC to follicular helper T cells (Tfh), and receive more survival signals (CD40L, IL-21). Lower-affinity B cells fail to compete and die. This selection pressure is what drives the accumulation of affinity-improving mutations.

Question 3

Repeated vaccination can improve the binding affinity of antibodies against a pathogen, not merely increase their quantity.

• T. True
• F. False

Answer: True

Each antigen exposure drives new germinal center reactions, including additional rounds of somatic hypermutation and selection. This produces memory B cells with progressively higher-affinity receptors. The antibodies from a booster dose are not only more numerous but qualitatively better binders — they dissociate from their antigen more slowly and neutralize pathogens more effectively. This is the mechanistic basis of vaccine boosters.

Question 4

Activation-induced cytidine deaminase (AID) specifically targets mutations toward the complementarity-determining regions (CDRs) of the antibody, ensuring that most SHM mutations directly affect antigen binding.

• T. True
• F. False

Answer: False

AID targets the variable region genes of immunoglobulin broadly, not specifically the CDRs. While there is some intrinsic hotspot preference based on sequence context (WRC motifs), mutations are distributed throughout the variable region — including framework regions that support structure rather than contact antigen. Most mutations are neutral or harmful. CDRs accumulate more affinity-relevant changes because selection, not targeting, filters for those that happen to improve binding.

Question 5

Why does affinity maturation require B cells to compete for limited antigen on follicular dendritic cells, rather than simply selecting any B cell that can bind antigen at all?

Model answer: Selection must distinguish between B cells of different affinities, not just between binders and non-binders.

If antigen were abundant, even low-affinity B cells would capture enough to present peptide to Tfh cells and receive survival signals — selection would be indiscriminate. By limiting antigen display on FDC surfaces, the germinal center forces B cells to compete: only those with the highest-affinity receptors capture sufficient antigen to present enough peptide-MHC and earn the Tfh survival signals (CD40L, IL-21) needed to avoid apoptosis. Scarcity is what makes selection stringent enough to drive the 10- to 100-fold affinity improvements observed over successive cycles.