Phagocytes engulf and destroy pathogens or foreign substances by the process of phagocytosis. Macrophages are scavengers found in the tissues and particularly in the lymph nodes, where they routinely sample the environment by engufing cells, particles, and large molecules. This process is non-specific as the macrophage usually cannot distinguish between foreign and self-structures. In order to do so, the macrophage must carry out the process known as antigen presentation. The breakdown products of digestion (antigen fragments) combine with the Class 2 major histocompatability complex molecule, and the larger complex (antigen-MHC) is brought to the cell surface. At the cell surface, the antigen-MHC complex is detected by the helper T cell. The neutrophil is by far the most abundant white blood cell. In addition to digestive enzymes, neutrophils contain reactive oxygen compounds that aid in the breakdown of bacteria and foreign materials.
Clonal Selection Theory
Scientists have developed the clonal selection theory to explain how lymphocytes respond to foreign antigens in a highly specific way. According to this theory, the body produces a large number of families (or clones) of cells, so-called because each family is derived from a common ancestor. Only those clones that have cell-surface receptor proteins that specifically fit the antigen can react with it, and are therefore "selected." T or B cells that react with antigen for the first time are called virgin or naÎve cells. NaÎve cells are stimulated to multiply and become either activated cells or memory cells. Activated cells are actively engaged in making a response (T cells carry out cell-mediated responses, while B cells secrete antibody). Memory cells do not make a response themselves, but are readily induced to become activated cells by a later encounter with the same antigen.
T Lymphocytes
Similar to all white blood cells, T cells are derived from stem cells in the bone marrow. Early during fetal development the cells migrate to the thymus gland where they differentiate and mature. Two types of T cells are formed (helper and cytotoxic) which differ by the nature of the antigen receptors on their surfaces. While developing, the T cells are exposed to MHC molecules of the thymus as well as from other parts of the body. In this manner, the T cells learn to recognize these molecules as self instead of as foreign antigens.
