Antigens are molecules that can be recognized and bound by the immune system, specifically by antibodies or immune cells, such as T cells. The term “antigen” is derived from “antibody generator,” highlighting their role in stimulating an immune response. Antigens are usually proteins or polysaccharides, but can also be lipids or nucleic acids.
Antigens can be classified into two main categories:
- Exogenous antigens: These are foreign substances that originate outside the body, such as components of bacteria, viruses, fungi, parasites, or allergens. Exogenous antigens can be introduced into the body through infections, vaccinations, or environmental exposure. The immune system recognizes these foreign antigens as non-self and mounts an immune response against them to neutralize or eliminate the threat.
- Endogenous antigens: These are self-antigens that originate within the body, such as proteins and other molecules found on the surface of cells. Under normal circumstances, the immune system recognizes endogenous antigens as self and does not mount an immune response against them. However, in certain cases, such as cancer or autoimmune diseases, endogenous antigens can become targets of the immune response.
Antigens play a critical role in the adaptive immune response, which is a highly specific and long-lasting defense mechanism against pathogens and foreign substances. When an antigen is encountered by the immune system, it is recognized and bound by antigen-presenting cells (APCs), such as dendritic cells, macrophages, or B cells. The APCs process and present the antigenic peptides on their surface in complex with major histocompatibility complex (MHC) molecules, which are then recognized by T cells.
This antigen presentation process leads to the activation and differentiation of T cells and B cells, ultimately resulting in the production of antigen-specific antibodies and the development of cellular immunity. This process is essential for the clearance of pathogens and the establishment of immunological memory, which provides long-term protection against future encounters with the same antigen.
