What is an antigen?

An antigen is defined as a substance that triggers an immune response in the body. Specifically, antigens are often foreign molecules such as proteins or polysaccharides found on the surface of pathogens, including viruses and bacteria. When introduced into the body, antigens are recognized by the immune system, which can lead to the production of antibodies and the activation of various immune cells. This response forms the basis of how the body defends itself against infections and diseases.

In contrast, the other options are distinct terms with different meanings in biology. For example, a molecule that binds to an enzyme refers to substances that can interact with enzymes to either inhibit or enhance their activity but does not inherently relate to immune responses. Likewise, a type of antibody is a specific protein produced by the immune system in response to an antigen, not an antigen itself. Lastly, while proteins can sometimes act as antigens, not all antigens are proteins. They can also be carbohydrate molecules or nucleic acids, among other substances. Thus, identifying the correct definition of an antigen as a substance that triggers an immune response is fundamental to understanding immunology.