In the context of the immune response, what is an antigen?

An antigen is defined as a molecule that can specifically bind to an immune receptor, which plays a crucial role in the immune response. Antigens are typically proteins, polysaccharides, or other biomolecules found on the surface of pathogens, such as bacteria and viruses, or even on infected cells. When an antigen is recognized by the immune system, particularly by antibodies or T cell receptors, it prompts an immune response, facilitating the identification and elimination of the foreign substance.

This specificity is key; the structure of the antigen dictates which immune receptors it can bind to, leading to the activation of various immune cells. The process ensures that the body can effectively target and respond to a wide range of pathogens and foreign molecules.

In contrast, while receptors on T cells are indeed involved in recognizing antigens, they themselves are not antigens. Similarly, while pathogens invade the body and may carry many antigens, they are not defined as antigens themselves. The enhancement of the immune response is related to different cells and molecules, such as helper T cells or cytokines, rather than being the direct definition of an antigen.