The structure of all immunoglobulins is basically the same, consisting of two heavy chains (Heavy, abbreviated as H) and two light chains (Light, abbreviated as L), and looks like a “Y”-shaped symmetrical macromolecule. The stem of “Y” is called the Fc segment, which consists of the -COOH group end half of the two heavy chain polypeptide chains. The F c segments of the two heavy chains are connected by one or several disulfide bonds. The two forks of “Y” are called F ab segments, and each fork is composed of a whole light chain and half of the -NH base end of the heavy chain. The distal end of each fork has an antigen binding site with the same characteristics. This structural model has been confirmed by electron micrographs. At the same time, it was also confirmed that the Fc segment and two Fab segments of a complete antibody molecule are connected by a hinge region. There are a lot of proline in the hinge area, the structure is extremely loose, and it has the characteristics of toughness and flexibility. It can adjust the angle between Fab segments to adapt to the spatial changes between antigenic determinants, so as to coat antigens more effectively.

For the same type of antibody, the most characteristic feature that constitutes each immunoglobulin molecule is expressed in the 110-120 residues of the -NH base end of the polypeptide chain. The amino acid sequence of this part is different from antibody protein to antibody protein. However, the rest of the antibody polypeptide chain is constant. Therefore, according to the amino acid sequence of the constant region, the light chain can be divided into two types, namely K (kappa) and λ (Lamda). Among them, K has no subtype, while the constant region of the λ type has a certain change. By analyzing the amino acid sequence at positions 1-20 of -NH2, it can be typed again. Among them, K is divided into I, II, and III, and λ is divided into I, H, I, W, and V. The K subgroup cannot be combined with the lambda type, and vice versa. Further research found that a plasma cell can only produce one type.

The heavy chain can also be classified according to the amino acid sequence of its constant region, which is divided into five categories: IgG (γ chain), IgM (μ chain), IgA (α chain), IgD (δ chain) and IgE (ε chain). Similar Ig Molecules are divided into several subclasses according to the position and number of disulfide bonds between chains. For example, IgG is divided into four subclasses, IgM is divided into two subclasses, and IgA is divided into two subclasses. There are 4 disulfide bonds in the γ and α chains, and 5 (or more) disulfide bonds in the μ, ε and δ chains. According to the characteristics of 1-20 amino acids at the base end of -NH, the heavy chain can be divided into four subgroups. The subgroups of the H chain are different from the subgroups of the L chain, and they can be combined with the constant regions of other heavy chains. For example, VHI can be combined with the constant region of α and γ.

The above classes, subtypes and types, subtypes and subgroups are all of the same genus, and the Ig specificity shared by all individuals is called isotype. The same species but different individuals, the polypeptide chain can have one or more amino acid differences, which is called allotype. This situation is very similar to the situation of ABO blood type and major histocompatibility antigen (H-LA). Generally speaking, Gm is the type of IgG. There are 30 such genotypes. The Gm factor is located at CH1-3. There is also the Am type of IgA, which is mainly determined by the α subtype, namely Am1-2. The difference lies in the presence or absence of disulfide bonds. Km is mainly different from the amino acid at position 191. For example, when it is leucine, it is called Km1; when it is valine, it is called Km3. Except the genotype of IgG is determined by the Fc segment, everything else is determined by the Fd segment. In addition, there is a so-called idiotype, which is determined by the antibody-producing cell. In the same genus, each individual produces different antibodies when stimulated by the same antigen, so there are countless idiotypes. Another notable feature of the polypeptide chains of the H and L chains is that each chain includes a homologous segment of approximately 110 residues.

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