B cells are involved in humoral immunity, due to their ability to secrete antibody into the blood targeting a specific antigen. However, B cells are only found in the lymphoid organs.
T cells are a form of cell-mediated immunity, directly interacting with the pathogenic cells in the body. By identifying the non-self antigen, T helper cells are able to activate antibody production on interaction with a complementary B cell. Cytotoxic T cells recognise foreign antigens, before inducing apoptosis by a similar mechanism to NK cells.
Antigen recognition by the different types of T cell is by the same mechanism. T helper cells use CD4 to stabilise the interaction between MHC and the T cell receptor, while cytotoxic T cells use CD8 to stabilise their interaction. B cells use B cell receptors to identify the antigen presented on T helper cells – this helps activate them and allows clonal expansion for antibody synthesis.
Differentiation of B cells occurs in the bone marrow, while T cells differentiate in the thymus. Due to thymic involution, the ability to differentiate T cells diminishes as an individual ages.