A lymphocyte (white blood cell) that develops in the bone marrow, matures in the thymus and expresses what appear to be antibody molecules on their surfaces but, unlike B-cells , these molecules cannot be secreted. This is called a T-Cell Receptor (CD3, and CD4 or CD8). Works as part of the immune system in the body. Produces cytokine to help B-Lymphocytes produce Immunoglobulin .

Several distinct T-cell subpopulations are recognized:

  Helper T-Cell (T4 cell, CD4 cell). A class of T-cells which help trigger B-cells to make antibodies against thymus-dependent antigens . Helper T-cells also help generate cytotoxic T-cells.

  T Killer Cells (cytoxic T-cells). Cells that kill target cells bearing appropriate antigen within the groove of an MHC class I molecule that is identical to that of the T-cell.

  T-suppressor cell (T8 cell, CD8 cell). The existance of these cells is a relatively recent discovery and hence their functioning is still somewhat debated. The basic concept of suppressor T-cells is a cell-type that specifically suppresses the action of other cells in the immune system, notably B-cells and T-cells, thereby preventing the establishment of an immune response. How this is done is not known with certainty, but it seems that certain specific antigens can stimulate the activation of the suppressor T-cells. This suppressor effection is thought to be mediated by some inhibitory factor, secreted by suppressor T-cells. It is not any of the known lymphokines . Another fact that renders the study of this cell-type difficult is the lack of a specific surface marker. Most suppressor T-cells are CD8 positive as are cytotoxic T-cells.