The T cell is one of the two major cell types of the ‘adaptive’ immune system. The other is the antibody-producing B cell. Adaptive means that the immune response adapts to the specific threat such as a virus. T cells express a cell surface protein called the T cell receptor (TCR). The TCR for each T cell is different due to genetic recombination. There are millions of different TCRs. When the immune system sees a new antigen from a virus, bacteria, or tumor, it screens these different TCRs for reactivity against the antigen. Once a T cell(s) responds, it expands and grows exponentially leading to millions of T cells that recognize that specific antigen, hence the term adaptive.There are two major kinds of T cells, the CD4 helper T cell and the CD8 cytotoxic T cell. The CD4 helper T cell is the central orchestrator of the immune response and stimulates antibody production and CD8 cytotoxic T cell activation. CD4 T cells recognize antigens released by tumor cells. These antigens are picked up by what are called antigen presenting cells (APCs). These APCs break the protein antigens into fragments called peptides. These peptides are then presented to the CD4 helper T cells attached to specialized proteins called HLA-class II molecules. Once activated, CD4 helper T cells activate other immune cells by releasing a number of proteins into tissues and the blood. In contrast to the CD4 helper T cell, the CD8 are cytotoxic which means that they directly kill the cells presenting tumor or virus antigens. They recognize peptides presented by HLA class I molecules, which are on all cells in the body.
Question by: JKJones