The immune system, composed of a variety of cell types and soluble factors, is geared toward the recognition of and response to "foreign" substances termed antigens. Vaccines convey antigens from living or killed microorganisms (or protein or carbohydrate molecules derived from these antigens) to elicit immune responses that are generally protective but can occasionally backfire and cause harm to the recipient. Specific immune responses, which interrupt the infectious process, generally take the form of immunoglobulin proteins called antibodies and/or activated immune cells that recognize particular antigens from an infectious agent. Immunity is medically induced by active or passive immunization
