The germinal center reaction can be divided into several phases:
1. Initiation Phase: During this phase, naive B cells that have encountered the antigen and received T cell help proliferate and form a primary focus. Some of these B cells enter the follicles to form germinal centers.
2. Dark Zone: In this zone, B cells called centroblasts undergo rapid proliferation and somatic hypermutation of their immunoglobulin genes. This process generates a diverse pool of B cell receptors with varying affinities for the antigen.
3. Light Zone: Centrocytes, which are derived from centroblasts, migrate to the light zone where they compete for binding to the antigen presented on the surface of follicular dendritic cells. The competition leads to the selection of B cells with higher affinity receptors.
4. Selection and Differentiation: B cells with high-affinity receptors for the antigen are selected for survival. These selected B cells can differentiate into either plasma cells, which produce and secrete antibodies, or memory B cells, which provide long-term immunity.
