Heterogeneity of antibodies. The composition of antibodies is extremely complex, consisting of thousands of diverse immunoglobulin (Ig) molecules. These Ig molecules are similar and different in shape, size, structure, and composition and arrangement of amino acids. Due to the difference, their electrophoretic activity will vary greatly.
Since the antibody has a binding site (antigen binding cluster) corresponding to the antigenic determinant, the binding of the antibody to the antigen is specific. On the other hand, the antibody itself is a protein, with its own amino acid composition, arrangement and three-dimensional structure. For heterogeneous animals, it is also an antigen. Various types of Ig have antigen specificity that can be detected by serological methods, and they show different serological types. There are three specificities of Ig antigens: ①Isotype specificity: the same type of antigen specificity common to all individuals. Human Ig can be divided into 5 categories (IgM, IgG, IgA, IgD, IgE), two types (lambda type and kappa type), and several subclasses, subtypes, groups and subgroups. However, the specificity of antibody-antigen binding has nothing to do with antibody class, subclass, type, etc. ② Allotype specificity: an antigenic determinant shared by certain individuals of the same genus (such as human). This is determined by genetics. In the constant regions of certain heavy and light chains (κ), individual amino acids are substituted to form Gm, Am, Km types; ③ unique type specificity: produced by a single B cell clone The unique antigenicity of Ig molecule. This determinant is in the variable region of the heavy and light chains, especially in the hypervariable region of the variable region. Since each individual antibody-forming cell is composed of polyclones, the unique type specificity is extremely high.
Any antibody in a normal human serum contains thousands, a wide variety, and a mixture of immunoglobulin molecules with various unique antigenicities. κ chain and λ chain can cooperate with various types and subtypes. The heavy chain and light chain itself have various types, and the Ig produced by each clone has its own unique type.
The diversity of antibodies is controlled by the genetics of the B cell system. The peptide chain is composed of two different genes encoding the variable region or the constant region. The constant region gene (C gene) is limited. Although it can determine the type and subclass of Ig molecules, it is the reason for the diversity of Ig molecules. One, however, the main reason for the diversity of immunoglobulin molecules is the heterogeneity of the variable region. The variable region is encoded by the V gene, and the number of V genes is still unclear.
There have been three doctrines of genetic control of antibody diversity: â‘ Germline doctrine (also known as germline doctrine): antibody-forming cells have all genes encoding Ig molecules (ie, a limited number of C genes and an unknown number of V genes, which It is formed through long-term evolution and passed from the parent to the offspring through the germ cell. Also known as the germline theory; â‘¡The somatic mutation theory: Only a limited number of V genes are inherited in the germ cell, and the formation of Ig molecular diversity is due to Somatic cell mutations or gene recombination during development, resulting in many different V genes. Somatic mutations may play an important role in the diversity of Ig molecules; â‘¢ V region gene interaction theory: Ig molecular variable region is composed of V genes The composition of fragments, J gene fragments and D gene fragments, the interconnection of V, D and J genes is extremely important for the generation of Ig molecular diversity. The diversity of Ig cannot be simply attributed to one of the above doctrines, it It may be related to the above-mentioned multiple mechanisms, and may also be related to the connection diversity at the connection points of gene fragments and the different pairing of VL and VH chains.
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