In immunology, what does the term "antigen" refer to?

In immunology, the term "antigen" refers to a substance that induces an immune response. Antigens are typically foreign molecules, such as proteins or polysaccharides found on the surface of pathogens like bacteria and viruses, or on the surfaces of allergens. When the immune system detects these antigens, it generates a specific immune response aimed at neutralizing or eliminating the invading organisms.

While antibodies are produced in response to antigens, they themselves are not categorized as antigens. The specific interaction between an antigen and an antibody is a fundamental aspect of the immune response, but the definitions of these terms are distinct.

Cells involved in the immune response, such as lymphocytes (T cells and B cells), interact with antigens but are not antigens themselves. Similarly, while proteins found in blood can include antibodies or other components of the immune system, not all of them are classified as antigens. Only those substances that provoke an immune response are designated as antigens. This makes the definition tied to the ability to stimulate immunity central to understanding the role of antigens in the immune system.