Principles of the Assay
T cells are so called because they are predominantly produced in the thymus. They recognise foreign particles (antigen) by a surface expressed, highly variable, T cell receptor (TCR). There are two major types of T cells: the helper T cell and the cytotoxic T cell. As the names suggest helper T cells ‘help’ other cells of the immune system, whilst cytotoxic T cells kill virally infected cells and tumours.
Unlike antibody, the TCR cannot bind antigen directly. Instead it needs to have broken-down peptides of the antigen ‘presented’ to it by an antigen presenting cell (APC). The molecules on the APC that present the antigen are called major histocompatibility complexes (MHC). There are two types of MHC: MHC class I and MHC class II. MHC class I presents to cytotoxic T cells; MHC class II presents to helper T cells.
T cell (left); Antigen presenting cell (right)
The binding of the TCR to the MHC molecule containing the antigen peptide is a little unstable and so co-receptors are required. The CD4 co-receptor (first image, below) is expressed by helper T cells and the CD8 co-receptor (second image, below) by cytotoxic T cells.
Cytotoxic T cell
The TCR is made up of multiple chains to assist the transmission of the signal to the T cell. These chains arealpha + beta OR gamma + delta. These are joined, in addition, by two epsilon chains, two zeta chains, and a delta and a gamma chain (confusing but don’t get the last two chains mixed up with the gamma and delta of the T cell receptor!). These are called, collectively, CD3 (see above).
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