Immune response decisions at the single cell level.

Helper T cells regulate the progressive differentiation of antigen-specific B cells in complex and dynamic microenvironments in vivo. In this review, antigen-driven B-cell differentiation will be discussed in three phases: pre-germinal center, germinal center and post-germinal center. Each phase of the response appears to have unique cellular starting points, specialized regional microenvironments and its own set of controls and cellular outcomes. Decisions made by individual lymphocytes at each of these phases determines the shape of protective immunity in vivo.