A T cell is activated when it recognizes the antigen that is bound by its receptor. This process involves sampling of antigens that are presented to it by antigen presenting cells. These cells are phagocytic cells that arrive at the site of tissue trauma and phagocytize the pathogens that may be present. During phagocytosis the pathogens are chopped to pieces and these pieces can be moved to the surface of the presenting cell and associated with MHC class II proteins. This complex is presented to helper T cells and the T cell that has a receptor that corresponds to the presented antigen becomes armed.
Although both types of lymphocytes are produced in the bone marrow, maturation of B cells occurs in the bone marrow but maturation of T cells takes place in the thymus. B cells bind to free antigen, whereas T cells require antigen to be presented bound to MHC molecules. Both types of cell require co-stimulatory molecules.
The cellular events involved in the generation of an adaptive immune response is a complex interaction between antigen presenting cells, T helper cells and B cells. There are two ways this can occur. The first involves the presentation of antigens to helper T cells by antigen presenting cells such as macrophages or dendritic cells. The other involves B cells that can present antigens to helper T cells. In either case if the presentation is proper the helper T cells will “instruct” the B cell to produce antibody against the antigen presented. This process involves the differentiation of the B cell into plasma cells which then produce antibody.