What is an antigen?

An antigen is defined as a substance that induces an immune response when detected by the immune system. This is generally because antigens are recognized by the body as foreign or non-self, prompting the immune system to react to eliminate or neutralize the invading entity. Antigens can be proteins, polysaccharides, and other molecules found on the surface of pathogens such as bacteria and viruses, as well as on the surfaces of cells, tumor cells, and allergens.

The immune response involves the activation of various immune cells, including B cells, which can produce antibodies specific to the antigen, and T cells, which help in directly attacking infected or malignant cells. The unique structure of an antigen is critical for the immune system’s ability to recognize and remember it, providing long-lasting protection against future infections from the same pathogen.

In contrast, while blood cells play critical roles in the immune system, they do not fall under the definition of antigens. Medications are used to treat diseases but are not naturally occurring substances that induce immune responses. Lastly, waste products in the blood refer to metabolic byproducts that the body needs to eliminate and have no role in immune responses. This distinction showcases the specific function of antigens in immunology.