The Evolution of the Immune Response

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Abstract
Immunglobulins of two different sizes and classes that had antibody activity were purified from immune turtle serum. The characteristics of the IgM molecules did not deviate from those of molecules of this immunoglobulin class in other Placoderm-derived species. The low molecular weight immunoglobulin differed from mammalian IgG in that it had a sedimentation coefficient of 5.7, a shortened heavy chain with a molecular weight of 38,000, and low carbohydrate content with almost undetectable levels of hexosamine. This immunoglobulin had agglutinin activity, bound homologous and heterologous complement, and was transferred to the turtle egg. On the basis of the structural and biologic properties of this immunoglobulin, it was hypothesized that the C-terminal constant region of the heavy chains present in mammalian IgG was lacking. The implications of the molecular requirements for expression of the biologic function assigned to the Fc fragment were discussed in relation to diversification in function of immunoglobulins during evolution.

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