Help is contact plus cytokines
CD4+ T cell help is not a single signal. Contact between CD40L on the T cell and CD40 on the B cell is required, and cytokines such as IL-2 and IL-21 shape the response. Without this help, B cells can bind antigen but do not efficiently enter germinal centers or undergo class switching, and the response remains short-lived.
What the germinal center produces
- Proliferation of antigen-specific B cells
- Somatic mutation of variable regions followed by selection of higher-affinity variants (affinity maturation)
- Class switching that changes antibody isotype and effector function
- Generation of memory B cells and long-lived plasma cells
Why recall is faster and stronger
Memory B cells and memory T cells persist after the infection clears. On re-exposure they respond with a shorter lag, greater magnitude, and better-matched specificity than naive cells, because the antigen-specific clones are already expanded and their receptors have been selected for affinity. This accelerated recall is the cellular basis of protection against reinfection, and it is the pressure that drives antigenic variation in viruses.