In the early 1960s, investigators using chicken embryos demonstrated that the bursa of Fabricius, a mass of lym-phatic tissue associated with the cloaca of birds, was one of the anatomic sites of lymphocyte differentiation. When this tissue was destroyed in the chicken embryos (by either surgical removal or administration of high doses of testos-terone), the adult chickens were unable to produce antibod-ies, leading to impaired humoral immunity. The chickens also demonstrated a marked reduction in the number of lymphocytes found in specific bursa-dependent areas of the spleen and lymph nodes. These affected lymphocytes were therefore named B lymphocytes or B cells. The bursa-equivalent organs in mammals (including humans) are the GALT and the bone marrow, where B lymphocytes differentiate into immunocompetent cells. Thus, the “B” refers to the bursa of Fabricius or the bursa-equivalent organs of mammals. Investigators studying newborn mice found that removal of the thymus results in profound deficiencies in cell-mediated immune responses. The rejection of transplanted skin from a heterologous donor is an example of cell-medi-ated immune response. Thymectomized mice demonstrate a marked reduction in the number of lymphocytes found in specific regions of the spleen and the lymph nodes (thymus-dependent areas). The areas of depletion differ from those identified after removal of the bursa of Fabricius in the chicken. These affected lymphocytes were therefore named T lymphocytes or T cells; T refers to thymus.
attracted to the site of mast cell degranulation, where they neutralize the effects of histamine. Thus, eosinophils are frequently seen in connective tissue at allergic or other hypersensitivity reaction sites.
Typically, however, both cellular and humoral immune systems are involved, although one system generally predominates, depending on the stimulus.
 Humoral (antibody-mediated) immunity is mediated by antibodies that act directly on an invading agent. These antibodies are produced by B lymphocytes and by plasma cells derived from B lymphocytes. In some diseases (e.g., tetanus), a nonimmune person can be rendered immune by receiving an injection of antibody purifed from the blood of an immune person or animal. The effectiveness of this passive transfer proves that it is the antibody that is responsible for the protection.
 Cell-mediated immunity is mediated by specific T lymphocytes that attack and destroy virus-infected host cells or foreign cells. Cell-mediated immunity is important in the defense against viral, fungal, and mycobacterial infections, as well as tumor cells. Cell-mediated immunity is also responsible for transplant rejection.
Helper T and cytotoxic T lymphocytes ( CTLs) recognize and bind to antigens that are bound to MHC molecules.
To understand how the specific immune responses (hu-moral and cell-mediated responses) are initiated, one must grasp the central role played by the helper and cytotoxic T lymphocytes. Helper T and cytotoxic lymphocytes act as the immune system “patrols.” Both kinds of lymphocytes have a T-cell receptor (TCR), a transmembrane protein whose exposed portion is on the T-cell membrane in close proximity to the CD3 marker (Fig. 14.3). The TCR recognizes antigen only when it is attached to “identification molecules,” the MHC molecules. In addition, helper T lymphocytes can only recognize an antigen when it is “presented” to them by cells called antigen-presenting cells (APCs). Cytotoxic T lymphocytes can only recognize antigen on other body cells such as those transformed by cancer or infected with a virus.
The two classes of MHC molecules display peptides on the surface of cells.