Aggressins may be defined as substances secreted by certain organisms under favorable conditions of growth, which have the property of inhibiting phagocytosis by a specific action on the leucocytes and reticulo-endothelial system. They may, therefore, facilitate the rapid development of normally sublethal infections of disease-producing organisms, resulting in death. The introduction of aggressins into the body causes the production of specific anti-aggressins. An anti-aggressive serum acts

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... ifically against the aggressin and does not destroy the disease-producing organism. It, however, allows the phagocytes to exert their normal powers of destroying the organisms. Aggressins are non-toxic products of bacterial metabolism, thought to be secreted by the bacteria during the period of their greatest activity. It is impossible to state definitely whether these substances are secreted products or endo-products liberated only on the death and lysis of the bacterial cell. In either case they are absolutely non-toxic substances and to produce their typical action of enhancing virulence must be associated with a certain minimum number of living bacterial cells. Aggressive substances were noted by Salmon and Smith (1884), who state "that the germs of such maladies are only able to multiply in the body of the individual attacked, because of a poisonous principle or substance which is produced during the