Figure 1. The first step in the generation of an antibody response is the uptake of immunogen by antigen-presenting cells (APC) (A). Complex antigens undergo proteolysis to form peptides, some of which are bound by major histocompatibility complex class II molecules and transported to the APC surface. Helper T cells that bear receptors capable of interacting with the peptide–class II complexes can then bind to the APC, an event that enables additional interactions through costimulatory molecules. These recognition events result in the transmission of activation signals to the T cell, one consequence of which leads to expression by the T cell of other costimulatory molecules on its surface (B). The activated T cell is now poised to respond to those B cells that display similar peptide–class II complexes on their surfaces, acquired as a result of internalization of the same immunogen through specific surface immunoglobulin receptors. It is this interaction between T cells and B cells that is termed ‘help’, and that results in triggering of the B cell to differentiate into a plasma cell capable of secreting antibody of the same specificity as that of the immunoglobulin receptor (C). Cytokines are also produced by each cell type shown which profoundly influence the type of immune response that is elicited.

Figure 2. Competition for antigen capture by B cells. (a) When B cells expressing surface immunoglobulin specific for the haemagglutinin (HA) molecule and for the neuraminidase molecule (NA) are confronted with intact influenza virus particles, the virus is predominantly captured and internalized by those specific for HA due in part to the greater relative abundance of this molecule on the virion surface. The internalized antigen is processed and presented by the anti-HA B cell to allow its interaction with helper T cells for expansion and differentiation. (b) When the same virus particle is solubilized, both HA and NA molecules are available for capture by surface immunoglobulin on B cells specific for these molecules.