The immune system

B cells and T cells. In normal circumstances the adaptive immune system, which consists of B cells and T cells, specifically recognises and eliminates infections and forms an immunological memory to protect the body from repeated infection. Although B cells and T cells have many similarities, they recognise their targets (antigens) in different ways. The B cell receptor (BCR), which becomes an antibody when secreted, recognises and binds to large molecules, interacting directly with pathogens and their toxic products. The T cell receptor (TCR) on the other hand only recognises foreign peptide antigens displayed on the surfaces of the body’s own cells. Tiny fragments of protein – peptides – are processed inside the host cell and presented on the surface of the cell in a cup-like molecule known as the major histocompatibility complex (MHC). In this way the host cell can advertise that it is, for example, infected by a virus and attract a T cell to kill the infected cell (Figure 1.1). 

Figure 1.1 Comparison of T cells versus B cells and antibodies versus T cell receptors (not to scale). Antibodies are produced by B cells in response to infection with pathogens such as viruses and bacteria. These antibodies can bind to live, intact pathogens through recognition of whole, intact antigens – usually proteins. Once bound, antibodies earmark their targets for recognition and destruction by other immune cells. As antibodies can bind to many biological molecules, they can also recognise ‘self-antigens’ and this property can be used for the treatment of many diseases such as leukaemia and lymphoma. T cells on the other hand possess a T cell receptor (TCR), which is a complex of proteins that interact with small peptide fragments derived from pathogens. These peptide fragments are presented to T cells in a grooved cup-like molecule called the major histocompatibility complex (MHC) for recognition by the TCR.