Simple Definition:

An antigen is a substance that triggers an immune response in the body, causing the production of antibodies to fight against it. Antigens can be foreign substances like viruses, bacteria, or toxins, or they can be self-antigens produced by the body’s own cells.

For example, a virus like the flu virus can be an antigen that triggers an immune response in the body. Similarly, the body’s own cells can produce self-antigens that may be recognized as foreign by the immune system, leading to autoimmune disorders [when the body attacks itself].

Very Simple Definition:

An antigen is something that makes your body’s defense system start fighting against it. It could be a germ or a toxin, or even a part of your own body that your immune system thinks is foreign.

For example, when you get sick with a cold, the virus that causes the cold is an antigen that makes your body produce antibodies to fight it off.