Cluster of differentiation antigens are uniquely expressed on the surface of all lymphocytes. CD antigens serve as biochemical markers characteristic for a particular cell type indicating the lineage or stage of maturity. CD antigens are designated CD1, CD2, CD3, CD4, etc. The functions of many of the 247 currently recognized CD proteins are still unknown.
The function and state of activation of different cell types can be determined by the expression or presence of CD antigens on the cell surface. CD3 is found on the surface of all T-lymphocytes, but not B-lymphocytes, while other CD antigens (e.g. CD19 and 20) are found on B-lymphocytes but not on T-lymphocytes. T-lymphocytes are sub-typed by the presence of CD4 and CD8 antigens.
Dendritic cells are antigen-presenting cells that patrol skin and other organs. Dendritic cells recognize and capture antigens. Captured antigens are processed and displayed on the dendritic cell surface. Following antigen capture, dendritic cells migrate to lymphoid tissues where antigens bound to the surface of dendritic cells are presented to lymphocytes in lymphoid tissue, activating the lymphocytes.
B-lymphocytes (B-cells) display a variety of receptors on their surfaces including:
- complement receptors
Glycoprotein structure is genetically determined and coded by major histocompatability complex (MHC) proteins.
There are two classes of MHC protein – Class I and Class II. MHC proteins are also known as human leukocyte antigens (HLA). MHC class I (MHC-I) proteins are found on all nucleated cells, while MHC class II (MHC-II) proteins are found mainly on macrophages, B-lymphocytes (B-cells), and dendritic cells.
When viruses or bacteria invade a cell, they are ingested and disassembled into amino acid fragments inside the cell much like our weapon destruction programs. Following disassembly, fragments are loaded onto an MHC-I or MHC-II platform and released to the cell surface for display (exocytosis).
Enormous numbers of B-cells circulate in the human body, but only about one in every 100,000 B-cells produced is capable of recognizing any one specific combination of amino acids originating from the invading antigen.
T-cells develop from immature lymphocytes in the thymus gland (“t” for thymus) after migrating there from the bone marrow.
T-cells may be found in the:
There are three types of T-cells:
- cytotoxic T-cells (TC cells)
- helper T-cells (TH cells)
- suppressor T-cells (TS cells)
Cytotoxic T-cells regulate the immune system by attacking cells that are malignant or defective, helper T-cells stimulate T-cell and B-cell activation, and cytotoxic T-cells kill foreign pathogens or infected cells, and suppressor T-cells suppress immune response.
T-cells contain receptors that recognize foreign or abnormal amino acid fragments bound to MHC proteins on the surface of self-cells (remnants of infection or abnormal processes). T-cell receptors are protein complexes that bind to MHC-antigen complexes on self cells displaying abnormal or foreign antigen fragments.
T-cells are activated when antigen fragments bind to T-cell receptors. Like antibodies, T-cell receptors are specific for only one antigen and are capable of distinguishing self from non-self. Once antigen attachment has taken place, the cell becomes specific for the bound antigen and clone cells are produced specific for the antigen