“…Generally, the IRMs may be divided into substances of endogenous origin, normally produced by the genome of the host and those of exogenous origin which are not products of mammalian genome, but may stimulate production of endogenous IRMs and modulate the immune response of the host (Roth, 1988). The capacity of tested endogenous (neuropeptides, hormones, cytokines, immunoglobulins, peptides) and exogenous IRMs (plant and microbial extracts, synthetic compounds, feed additives, drugs) to improve immune status of laboratory and domestic animal species has been thoroughly reviewed (Wybran, 1988;Georgiev, 1991Georgiev, , 1993Valpotić, 2000). The synthesis of all these classifications, supplemented with newly emerged bioactive organisms/substances is given in the …”