What are antigens?

Antigens are defined as substances that provoke an immune response. They can be composed of proteins, polysaccharides, or other molecules that are recognized by the immune system as foreign. When antigens enter the body, they trigger the activation of immune cells, leading to a cascade of responses designed to neutralize or destroy the invading pathogen. Antigens can be found on the surface of pathogens like bacteria and viruses or in allergens and even in transplanted tissues.

Other choices describe different aspects of the immune system but do not accurately define antigens. For instance, vitamins play a role in overall health and support immune function, but they do not serve as antigens. White blood cells are critical components of the immune response and are responsible for targeting and eliminating pathogens; however, they are not classified as antigens themselves. Lastly, proteins produced by the immune system, such as antibodies, are responses to antigens rather than antigens themselves. The correct understanding of antigens is essential for grasping how the immune system recognizes and responds to various foreign substances.