What is an antigen?

An antigen is best defined as a molecule recognized by the immune system, specifically by antibodies and T-cell receptors. These molecules can be found on the surface of pathogens such as bacteria, viruses, and fungi, or they can be part of foreign substances like pollen or transplanted tissues. When an antigen is detected, it triggers an immune response, leading to the production of antibodies or the activation of immune cells that target and neutralize the perceived threat.

Understanding the properties of antigens is critical in immunology. They are not limited to proteins; antigens can encompass a wide range of molecules, including carbohydrates and lipids, so a more restrictive definition pertaining only to proteins would be inadequate. Furthermore, antigens are distinct from antibodies, which are the immune molecules produced specifically in response to antigens. This distinction illustrates the interactive nature of immune responses, where antigens are the stimuli and antibodies are the response, ensuring that the body can defend itself against infections effectively.