matter, substance - that which has mass and occupies space; "an atom is the smallest indivisible unit of matter"

immunizing agent, immunogen - any substance or organism that provokes an immune response (produces immunity) when introduced into the body

agglutinogen - any substance that acts as an antigen to cause agglutinin production

fetoprotein, foetoprotein - any of several antigens that occur naturally in the fetus and sometimes in adults with cancer

anatoxin, toxoid - a bacterial toxin that has been weakened until it is no longer toxic but is strong enough to induce the formation of antibodies and immunity to the specific disease caused by the toxin; "diphtheria toxoid"

Rh, Rh factor, rhesus factor - a blood group antigen possessed by Rh-positive people; if an Rh-negative person receives a blood transfusion from an Rh-positive person it can result in hemolysis and anemia