1.1 Cytotoxic T cells

T cells recognise antigens through their T cell receptor (TCR). To be precise, T cells recognise antigen fragments presented to them by cell-surface molecules encoded by the major histocompatibility complex (MHC molecules), shown in Figure 1. There are in fact, two types of MHC molecule – class I and class 2. Here we will concentrate on MHC class I molecules. All cells of the body continuously sample their own internal proteins and present polypeptide fragments on the cell surface bound to MHC class I molecules.

Internal molecules of the cell, which can be any intracellular molecule including a viral antigen (blue hexagon) are broken down into peptide fragments. These antigenic peptides are presented by MHC molecules on the cell surface, where they may be recognised by T cells that have an appropriate T cell receptor. (Note: in Figure 1 and later diagrams, the scale of the cells and the receptor molecules will differ.)

Figure 1 MHC molecules

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Internal molecules of the cell (blue hexagon) are broken down into peptide fragments. These antigenic peptides are presented by MHC molecules on the cell surface, where they may be recognised by T cells that have an appropriate T cell receptor. (Note that in this and later diagrams, the scale of the cells and the receptor molecules is completely different.)

Figure 1 MHC molecules

If the cell has become infected by a virus, then polypeptide fragments of viral proteins will also be presented by the MHC class I molecules. If a cytotoxic T cell recognises the antigen fragment+MHC molecule, then it can signal to the infected cell to induce apoptosis – programmed cell death. You will learn more about this later, but first you will look in a bit more detail at the MHC molecules.