What is an antigen?

An antigen is best understood as a foreign invader that stimulates an immune response in the body. Antigens are typically found on the surface of pathogens such as bacteria, viruses, fungi, and even certain cancer cells. When the immune system detects these foreign substances, it recognizes the antigens and mounts a response, often involving the production of antibodies that target that specific antigen.

This distinction is crucial in immunology, as recognizing a foreign invader is the first step towards an effective immune response. The body's defense mechanisms, including various types of immune cells, work to eliminate these antigens. While immune cells are involved in the process and blood proteins can also play roles in the immune response, they are not classified as antigens themselves. Similarly, hormones or hormone-like chemicals have different functions and are not defined as antigens. Understanding the role and definition of antigens is fundamental, especially in the context of vaccinations and immune responses to infections.