How are antigens defined in the immune response?

• A. Substances recognized as "self"
• B. Substances not recognized as "self" that provoke an immune response
• C. Only proteins that evoke a cellular response
• D. Body's own molecules that trigger allergic reactions

Answer: (B)

Antigens are defined as substances that are recognized as foreign by the immune system and provoke an immune response. This includes a wide variety of molecules such as proteins, polysaccharides, lipids, and even nucleic acids that are not part of the host organism (the "self"). The immune system’s ability to differentiate between self and non-self is crucial for its proper functioning; when non-self substances are detected, the immune system activates and mounts a response to eliminate them. This response can involve various immune cells and processes, including the production of antibodies.

In case of allergic reactions or autoimmunity, the immune system mistakenly identifies self-components as harmful, but these scenarios fall outside the traditional definition of antigens. The ability to recognize and respond to antigens is fundamental to the effectiveness of the immune system and its ability to protect against pathogens and other foreign invaders.