When a host is immunized, he/she has specific recognition and defense mechanisms against a particular foreign substance (antigen) which may be developed in a natural or artificial way. This immunity can be obtained both actively or passively. If the defense products originate in an endogenous site by means of the antigenic stimulation of the host immune system, the immunity is active. If, on the other hand, the antibodies or cells are produced in another host, by effect of the antigen stimulation, and are then transfer ed to the receptor, the immunity conferred is passive.