What are germinal centers and what occurs there during the B cell response?

• A. Germinal centers are sites of T cell activation.
• B. Germinal centers are where NK cells mature.
• C. Germinal centers are where plasma cells reside.
• D. In germinal centers, B cells mature through affinity maturation and class switching.

Answer: (D)

Germinal centers are specialized microenvironments within lymphoid tissues that form during a B cell–dependent immune response. Here B cells proliferate and undergo affinity maturation by somatic hypermutation of their immunoglobulin variable-region genes, with selection favoring higher-affinity receptors in the light zone. They also undergo class switching, changing their antibody isotype (for example from IgM to IgG, IgA, or IgE) under cytokine influence and help from T follicular helper cells, which alters effector function without changing antigen specificity. The combined outcome is generation of high-affinity, isotype-switched antibodies and the formation of memory B cells and long-lived plasma cells. These functions distinguish germinal centers from sites dedicated to T cell activation, NK cell maturation, or mere plasma cell residency.