Antibodies occurring naturally or normally in sera are seldom if ever as resistant to deleterious influences as immune antibodies, those produced by the body cells during active immunization; accordingly, the natural agglutinins and hemolysins in human sera for the erythrocytes of persons and of the lower animals are as a group thermolabile or heat sensitive and likewise highly susceptible to the deterioration of age and desiccation (1). Some of the natural hemolysins and hemagglutinins and particularly those for sheep corpuscles are more resistant than others, but the isoagglutinins and isohemolysins, which are of particular interest in relation to blood transfusion and the grouping of bloods, are quite sensitive. As a general rule the natural hemolysins are more susceptible to destruction by heat, age and desiccation than the agglutinins and in human serum containing an agglutinin and hemolysin for a particular group of corpuscles, the hemolysin disappears before the agglutinin.

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