Each antigen binds to a specific antibody, much like a key that can open a specific lock

An antigen is a molecule on the outside of a pathogen (a "germ"). It causes the production of antibodies which stick to the invading virus or bacterium. This is an immune response.

Antigens are usually a protein on the outside of a bacterium or virus. It gets recognized by the adaptive immune system as foreign, and this stimulates the production of antibodies. The antibodies combine with the antigens and make the invading virus or bacterium ineffective. Antigens stimulate the production of antibodies: they do not produce them directly. Vaccines for the seasonal flu virus is a common example.