What are antigens?

• A. Waste products of the immune response
• B. Substances that induce an immune response
• C. Cells that carry out immune functions
• D. Proteins that inhibit inflammation

Answer: (B)

Antigens are substances that induce an immune response, making the selected answer the most accurate choice. They can be proteins, polysaccharides, or other molecules found on the surface of pathogens, such as bacteria and viruses, as well as on non-pathogenic cells and substances, like pollen or transplanted tissues. When the immune system recognizes an antigen as foreign, it triggers a cascade of responses, including the activation of lymphocytes that produce antibodies specifically designed to target that antigen.

Understanding that antigens are critical for the activation of the immune system clarifies their role in both recognition and the initiation of immune reactions. This is essential not only in infections but also in vaccines, where harmless forms of antigens stimulate the immune system to develop memory against pathogens without causing disease.

The other options, while related to the immune system, do not accurately define antigens. Waste products of the immune response refer to the byproducts of immune activity rather than the substances that trigger the responses themselves. Cells that carry out immune functions describe components of the immune system, such as T cells and B cells, but do not explain what antigens are. Proteins that inhibit inflammation might describe certain regulatory proteins in the immune system but are not representative of antigens' primary function.