Epitopes are protein fragments recognized by the immune system, activating T and B cells and antibodies. T cells respond to smaller epitopes processed by antigen-presenting cells, while B cells and antibodies recognize larger epitopes in their normal conformations. Epitope mapping helps identify targets for vaccines and diagnostic tests.
An epitope is the part of a protein recognized by the immune system. They are recognized by specific T cells, B cells and by the antibody produced by the B cells. When these cells recognize and are activated by specific epitopes, they begin to mount an immune response. Most epitopes are derived from proteins that the immune system classifies as non-self, meaning the protein is part of a foreign organism such as a virus or bacterium.
T and B cells respond to different types of epitopes. Both cell types can respond to epitopes from the same organism, but do not respond to exactly the same epitope. This is because each cell type responds to epitopes of different sizes and shapes. In general, T cells react to epitopes that are much smaller than those recognized by B lymphocytes and antibodies.
For T cells, the activating epitope is a fragment of a protein. T-cell activating epitopes are those that have been processed by immune cells called antigen-presenting cells or APCs. When an APC processes epitopes, it does so by ingesting bacteria or viruses and breaking those organisms down into their constituent proteins and other macromolecules. The APCs therefore show fragments of proteins, which can be as small as eight amino acids in size, on their surfaces. When a T cell recognizes an epitope and receives other necessary stimulatory signals, the T cell becomes an active part of the immune response.
In contrast, B cells and the antibodies they produce are activated by much larger epitopes. The B cell and antibody epitopes are still in their normal three-dimensional conformations. This difference in the types of epitopes recognized by T cells and B cells allows each cell type to perform different immune functions.
Antibodies recognize epitopes on native proteins, which are often visualized on the surface of the specific infecting organism. The bacterium or virus eventually becomes coated with antibodies, making it vulnerable to being killed by phagocytes. In contrast, T cells recognize shorter epitopes and are primed for several functions, such as cytokine production. They are also efficient in killing cells infected by intracellular viruses or bacteria and recognize infected cells because the infection causes the cells to display foreign epitopes on their surfaces.
The strong significance of epitopes in the immune system has led to the development of epitope mapping. This technique was developed to try to identify elements of human pathogens that can be recognized as epitopes by T and B cells. Epitopes recognized as such can then be used as targets for vaccines and can also become the basis of diagnostic tests.
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