T Cell-Dependent B Cell Activation

Production of high-affinity, long-lived, circulating antibodies to vaccines or infectious agents or their products requires collaboration between two kinds of lymphocytes: antigen-specific helper T lymphocytes and antigen-specific B lymphocytes. The T cells provide the signals that the B cells need to proliferate and differentiate into clones of antibody-secreting plasma cells, and memory B cells. During birth in the bone marrow, each B cell assembles a gene encoding a unique antibody molecule, which it puts on the cell surface as a receptor for antigen. Antigen binding to the receptor on those rare B cells that are specific for it delivers activating signals to the B cell, but these signals are typically insufficient for an antibody response. The surface receptor also enables the antigen-specific B cell to internalize the antigen, process it, and present it back on the cell surface as a small peptide bound to an MHC class II molecule as a target for the helper T cell. When the helper T cell recognizes antigen presented by the B cell, it delivers help to the B cell in the form of secreted cytokines and an essential membrane-bound cytokine in TNF family called CD40 ligand (CD40L or CD154).