An antigen is a protein, which is capable of stimulates the production of antibodies and they do not create them directly. It is also a molecule, which is possibly on the surface of cell of a bacterium or virus. If an antigen is in the body of human, it may causes the making of antibodies. It contains bacteria, cells of transplanted organs, plant pollen, and toxins. Antigens are generally peptides (amino acids chains), polysaccharides (simple sugar) or lipids.

The word antigen, which is firstly described an important molecule binds especially to an antibody. It might be creating inside the body (self-antigen) or from the outside environment (non self-antigen).

An immunogen is an antigen substance, which is capable of activating a humoral or cell-mediated immune response. It starts an inborn immune response, which may causes the activation of the adaptive immune response. They are normally proteins and polysaccharides, and lipids. Lipids and nucleic acids are antigenic if they combined with proteins and polysaccharides. Non-microbial non self-antigens contain egg white, pollen, and proteins from transplanted tissues and organs or on the surface of transfused blood cells. The examples of antigens are vaccines, which are used to induce the memory function of adaptive immune system.