There are two major classes of lymphocytes involved with specific defenses: B cells and T cells.
Immature T cells are produced in the bone marrow, but they subsequently migrate to the thymus, where they mature and develop the ability to recognize specific antigens. T cells are responsible for cell-mediated immunity.
B cells, which mature in the bone marrow, are responsible for antibody-mediated immunity. The cell-mediated response begins when a pathogen is engulfed by an antigen-presenting cell, in this case, a macrophage. After the microbe is broken down by lysosomal enzymes, antigenic fragments are displayed with MHC molecules on the surface of the macrophage.
T cells recognize the combination of the MHC molecule and an antigenic fragment and are activated to multiply rapidly into an army of specialized T cells.
One member of this army is the cytotoxic T cell. Cytotoxic T cells recognize and destroy foreign cells and tissues or virus-infected cells. Another T cell is the memory cytotoxic T lymphocyte, which remains in reserve in the body. If, sometime in the future, these T cells re-encounter this specific antigen, they will rapidly differentiate into cytotoxic T cells, providing a speedy and effective defense.
Helper T cells coordinate specific and nonspecific defenses, in large part by releasing chemicals that stimulate T cell and B cell growth and differentiation.
Suppressor T cells inhibit the immune response so that it ends when the infection has been controlled. Whereas the number of helper T cells increases almost at once, the number of suppressor T cells increases slowly, allowing time for an effective first response.