The division of biological science concerned with the native or acquired response of complex living organisms to the intrusion of other organisms or foreign substances. The immune system allows the host organism to distinguish between self and nonself and to respond to a target (termed an antigen). An immunological response is specific and can be genetically determined or learned, that is, developed as a consequence of exposure. Examples of immune responses are neutralization of toxic protein molecules, killing of infecting microorganisms, rejection of foreign tissue grafts, and memory by the protective effects of vaccination. The last, discovered by E. Jenner toward the end of the eighteenth century, was the primary immunological finding.