An antigen's a substance that prompts the generation of antibodies and can cause an immune response. The word originated from the notion that they can stimulate antibody generation. We now know that the immune system doesn't consist of only antibodies. The modern definition encompasses all substances that can be recognized by the adaptive immune system. In the strict sense, immunogens are those substances that elicit a response from the immune system, whereas antigens are defined as substances… (More on Antigens)