immunological ontogeny
Immunological ontogeny
The origin and development (ontogeny) of the lymphocyte system, from its earliest stages to the two major populations of mature lymphocytes: the thymus-dependent or T lymphocytes, and the thymus-independent or B lymphocytes. The T lymphocytes carry out those aspects of function which are called cell-mediated immunity, including graft rejection, elimination of tumor cells, and delayed hypersensitivity. B cells are responsible for humoral or antibody-mediated immunity. See Cellular immunology, Immunity
For both systems, development or differentiation proceeds in discrete stages. In the first stage, pluripotent hematopoietic stem cells, which originate in the yolk sac in the embryo and then successively in fetal liver and fetal bone marrow, develop into precursor cells committed to becoming T or B cells. Hematopoiesis in human fetal liver begins at about 4 weeks of gestational age and in fetal bone marrow after 20 weeks. See Hematopoiesis
The thymus plays a strategic role in the development of T lymphocytes. Precursor cells are attracted into the thymus where, under the influence of this microenvironment, they undergo rapid proliferation and maturation. These maturing T cells also begin to express a variety of cell-surface markers, which parallels developing immunocompetence. From the thymus, the maturing T cells are exported to the peripheral lymphoid tissues. See Thymus gland
The earliest B cells identified in fetal liver are pre-B cells. As the cells mature, they express immunoglobulin M (IgM) and subsequently IgD on their surface. At this stage, the cell is a specific, competent B cell ready to interact with an antigen. In the course of B-lymphocyte differentiation, diversity of immunoglobulin classes is generated by an orderly switch from IgM to IgG to IgA with expression of the respective immunoglobulin on the cell surface. See Antigen, Immunoglobulin