Antigens are substances that the immune system recognizes as foreign or non- self and that can provoke an immune response. They can be parts of pathogens (like proteins on a virus or bacteria), allergens, or even abnormal or transformed cells, including transplanted tissues or cancer cells. When an antigen is detected, immune cells (such as B cells and T cells) respond by producing antibodies or launching cellular immune responses to neutralize or eliminate the threat. Key points

• Types: antigens can be exogenous (originating outside the body, e.g., pathogens), endogenous (originating inside the body, e.g., certain cellular abnormalities), allergens (triggering allergic reactions), or autoantigens (self-components that can be mistaken for foreign, as in autoimmune diseases).

• Structure: many antigens are proteins or polysaccharides, but they can also be lipids or nucleic acids. The specific part of the antigen that is recognized by an antibody or T-cell receptor is called an epitope.

• Function: antigens are recognized by the immune system, which then generates targeted responses (antibody production by B cells and activation of T cells) to eliminate or neutralize the threat.

• Clinical relevance: antigen concepts underpin diagnostic tests (such as antigen tests for infections), vaccine design (training the immune system to recognize specific antigens), transfusion compatibility, and autoimmune considerations.

If you’d like, I can tailor this with examples (e.g., specific viral or allergen antigens), explain how antigens differ from antibodies, or dive into how the body identifies epitopes on complex pathogens.