rather also an organ 2 system for specific and unspecific defense mechanisms, di­ rected against the hazards of mechanical, microbiological and toxicological risks posed to multicellular macro-orga­ nisms in an environment in which trauma and other distur­ bances of the physical integrity are common. The two defense systems of the blood are based on humural and cellular constituents; the latter have to be considered as "excitable cells", capable of responding in a predictable and automatic fashion to adequate stimuli. The response of the cellular constituents in controlled and coordinated by chemical mediators. The immunological research of the last century has distinguished two separate defense systems of the blood: I. The highly specific defense system (immune system sen­ sustrictori) which is phylogenetically new and is ontogenetically learned.