Antigen

From Memory Alpha, the free Star Trek reference.

An antigen is a substance or molecule that elicits an immune response. Antigens are frequently peptides or carbohydrates derived either from the host ("self-antigens" or "autoantigens") or from pathogens such as bacteria or viruses. Antigens are presented on the surface of cells by specialized protein complexes in a manner that elicits either an antibody response or a cytotoxic effector cell response.

Doctor Julian Bashir administered an antigen to several Teplan patients in late 2372, hoping it would help cure the Teplan blight. Although it was later determined injecting patients with antigens would not cure them of the Dominion-created disease, the antigen was able to allow the children of the Teplans to be immune, as the antigen was absorbed by the placenta, and could prevent them from developing the blight. (DS9: "The Quickening")

In 2377, the Doctor injected Administrator Chellick of the Dinaali Hospital Ship 4-2 with chromovirus, as well as antigens from Tebbis, a low-TC patient who had died from that disease. This caused the Allocator to read Chellick as Tebbis and deny him cytoglobin, forcing him to change his policies regarding medical allocation. (VOY: "Critical Care")