An antigen is any substance that is capable of triggering an immune response in an organism. Antigens are typically proteins or large polysaccharides, although other types of molecules can also act as antigens. These substances are recognized by the immune system as foreign or non-self, which initiates an immune response aimed at neutralizing or eliminating them from the body.
When an antigen enters the body, specialized cells of the immune system, such as B cells and T cells, recognize and respond to it. B cells can produce antibodies, which are proteins that specifically bind to antigens and help neutralize or eliminate them. T cells, on the other hand, can directly attack cells that are infected or abnormal due to the presence of antigens.
Antigens can be derived from various sources, including pathogens such as bacteria, viruses, fungi, and parasites. They can also come from non-infectious sources such as allergens (substances that induce allergic reactions), toxins, and certain cells or tissues from other organisms (in the case of transplantation or transfusion).
Understanding antigens and their interactions with the immune system is crucial for vaccine development, diagnostic testing, transplantation medicine, and immunotherapy treatments for various diseases.