B Cell Activation and Class Switching

Expansion

1. Antigen binds the B cell receptor (membrane IgM and IgD)
2. T cell help via CD40 ligand and cytokines
3. Germinal centre reaction in the lymph node, where two processes occur:
   • Somatic hypermutation and selection, giving affinity maturation
   • Class switching from IgM to IgG, IgA or IgE, determined by the cytokines present
4. Differentiation into plasma cells (antibody factories) and memory B cells

Primary response: lag of 5 to 10 days, mainly IgM, low affinity, short lived.

Secondary response: lag of 1 to 3 days, mainly IgG, higher affinity, much larger and more durable. This is the basis of booster vaccination.

T-independent antigens, chiefly bacterial polysaccharides, activate B cells directly but without T cell help. The result is IgM only, no affinity maturation and no memory, and poor responses in children under 2. This is exactly the deficiency that conjugate vaccines overcome by attaching the polysaccharide to a protein carrier.