Antibody structure.

Abstract

A consideration of the overall properties of antibodies has for some time suggested that their structural features must be unique among protein molecules. In the first place, serum antibodies within a species share a variety of physical, chemical and antigenic properties indicating that all have a common basic structure. However, the immunoglobulins of all animal species investigated occur in several forms, somewhat arbitrarily designated as classes, subclasses, or types, and distinguished on the basis of their chemical and biological properties. In addition, the great diversity of antigens which evoke an immune response and the narrowly defined specificity of the serum antibodies formed indicate that each individual can synthesize a very large number of antibodies-probably more than 105and there is conclusive evidence that these differ from one another in their covalent structure. The fundamental problem of antibody structure therefore concerns the nature of the molecular modifications which are superimposed upon a relatively constant basic configuration to generate a finite number of immunoglobulin classes and a very large assortment of distinct combining specificities. The extent to which this problem has been elucidated by studies of protein structure is outlined in this paper. It has been reviewed by Leslie and Cohen (1973). There is an additional intriguing problem which concerns the way that antibodies mediate complex biological reactions including, for example, the expression of acquired protective immunity or immediate hypersensitivity. There are very few instances in which the interaction of antigen with antibody leads directly to biological effects. Such primary reactions are limited to the inactivation of enzymes and toxins and the neutralization of some viruses and protozoa. In the great majority of cases the biological expression of immune reactivity requires interaction of antibody with either the complement system or with specific cell surfaces. Such reactions involve specific effector sites on the antibodies which are distributed unevenly among the classes and subclasses of immunoglobulin. These sites are involved in complementor cell-mediated lysis, opsonization by macrophages and mast cell