Lymphocytes comprise about 30% of the circulating leukocytes. Lymphocytes are involved in the development of adaptive or acquired immune responses. There are two major types of lymphocytes: T-cells and B-cells, both having surface receptors for antigen.
The Antigen-Antibody Reaction
When an antigen enters the body, two types of adaptive immune responses can occur:
The synthesis and release of free antibody into the blood and other body fluids, called Humoral Immunity, is provided by B-cells.
The production of sensitized lymphocytes called T-cells that are effectors of Cell-mediated Immunity.
Cellular Immunity involving T-cells is effective against fungi, many parasites, intracellular bacteria, most viruses, cancer cells, and surgically transplanted or transfused foreign tissues. This is the type of response associated with graft rejection in transplant cases, and also with transfusion incompatibility.
Humoral Immunity, through circulating antibodies, is effective against extracellular organisms, including bacteria, some parasites, and some viruses.
Lymphocytes are produced in bone marrow from stem cells. A portion of the lymphocytic precursor cell population migrates to the thymus to mature into T-cells, while others are processed in the bone marrow to become B-cells. It should be mentioned that at 8-9 weeks of fetal development, B cells form in the liver but, soon after, the bone marrow becomes the primary site of production. The thymus gland and bone marrow are considered primary lymphoid organs while peripheral lymph nodes, mucous associated lymphoid tissue and the spleen are considered secondary lymphoid organs.
MALT is a significant component of mucosal immunity. It is the largest mammalian lymphoid organ system and comprises approximately 80% of all lymphocytes.