An antibody is a protein molecule produced by the immune system to mark foreign viruses and bacteria within the body, usually for attack.
The presence of specific antibodies can be used to test for the presence, now or in the past, of specific viruses or bacteria.
During the 2350s and 2360s, experimental research at the Darwin Genetic Research Station resulted in a group of human children having antibodies aggressive enough to destroy pathogens even before they could invade a human body. These antibodies were lethal to humans who lacked them, and all aboard the USS Lantree were killed in 2365 as a result of this. (TNG:"Unnatural Selection").
In 2371,The Doctor fashioned antibodies to Parinisti measles, hoping to create a vaccine (VOY: "Heroes and Demons").
In 2374, Annorax launched a temporal incursion intended to restore the Krenim Imperium to power, creating an alternate timeline wherein the Rilnar were destroyed by the Krenim before the Rilnar could introduce a vital antibody into the Krenim genome, resulting in a plague (VOY: "Year of Hell, Part I").