Antibodies are a key part of the immune system. When a foreign particle or substance, like a virus or bacteria, enters the body, specialized cells at the entry site see that certain proteins on the substance are foreign. These foreign proteins are called antigens.
In the skin, these specialized "watchdog" cells are the langerhans cells. The langerhans cells present the foreign particle to the immune system which makes proteins that attach to the antigens. These proteins are antibodies and they're very similar to the note you secretly tape to someone's back that says, "Kick me!" When other cells in the immune system see the antibodies, they kill the antigen.
After the immune system has eliminated the foreign substance, some of the antibodies stay around. If a person is later exposed to the same antigen, the antibodies quickly attach and cause many more antibodies to be formed, hopefully killing the infectious agents before they can hurt the body.
Sources:
"Antibody." Dorland's Medical Dictionary. 2007. Elsevier. 15 Sep 2008
"Antibody. "Mosby's Medical, Nursing, and Allied Health Dictionary. 6th ed. 2002.
