Foreign organisms such as bacteria invade the normal tissues of the body, or your body's own cells start reproducing out of control, becoming cancerous. The surface of these bacteria or abnormal cells bristles with antigens, which spur the immune system into action, much as a burglar alarm sends a signal to the police station. As these antigen-covered cells travel through the body, they're swept up into immune system organs such as the spleen and lymph nodes.
There they swirl about in a sea of specific fighting proteins called antibodies. Antigens and antibodies brush against each other constantly in this huge crowd. When the antibodies and their specific antigens find each other, they interlock. This locking stimulates the antibody-producing B cells to pump out millions of antibodies that travel through the body, meeting and destroying the specific antigen intruders.