Structure of IgG "👇🏻

 BASIC STRUCTURE OF IgG "👇🏻

Electron microscopic studies have revealed that IgG is a Y-shaped tetrapeptide protein formed of two identical light chains and two identical heavy chains. So an antibody is represented as H₂L₂

(1) Light Chains. Each light chain is formed of about 214 amino acids. Light chains are of two types in all the classes of immunoglobulin: Kappa (к) and lambda (λ.) chains. A given antibody has either both Kappa or both lambda type light chains.

The amino acid sequence analysis of light chains has shown that a light chain is formed of 2 parts: Variable (V) of 1-108 amino acids and varies from one antibody to another; and Constant (C) of 109-214 amino acids and is characteristic of the antibody. Each light chain also has two intrachain disulphide (S-S) loops, one in variable part and other in constant part.

(2) Heavy Chains. These are two in number. Each heavy chain is formed of 440 amino acids. Each heavy chain is formed of two parts: Variable region (formed of 1-118 amino acids) and Constant region (formed of 119-440 amino acids and about three times in length to that of light chain). The constant region of each heavy chain has three intrachain -S-S-loops. Similarly, the variable region also has a -S-S-loop. So each heavy chain of IgG has four -S-S-loops.

The part of the hypervariable regions on the antibody which binds the antigen is called the paratope, while the corresponding part of the antigen which is in contact with the paratope is called the epitope.

Two light chains and two heavy chains of IgG are interlinked through -S-S- (disulphide) bonds. There are four disulphide bonds in IgG.

* ANTIGEN ANTIBODY INTERACTIONS : 👇🏻

Each antibody is specific for a particular antigen. Antigen-antibody interactions involve : 

Structure of an antibody molecule (IgG): 👇🏻

Combining sites of antigen, called antigenic determinants, react with the corresponding binding sites of antibodies which lie at the N-terminal end of the antibody molecules. Antigen binds with both light and heavy chains of the antibody. Ag-Ab intermolecular forces include electrostatic forces of attraction, reversible H-bonds, hydrophobic bonds and Van der Waals forces of attraction.

These antibodies fight the antigens in five different ways:

(a) Neutralization. Some antibodies neutralize the antigens, termed toxins (viral toxins, bacterial toxins, snake venom), and make them ineffective. They are called neutralins or antitoxins. The phagocytes dispose of the neutralized antigen-antibody complexes.

 (b)Agglutination : Certain antibodies cause the particulate antigens (bacteria, red corpuscles) stick together in clumps, thus immobilizing them for easy disposal by the phagocytes through ingestion. They are termed agglutinins.

(c)Precipitation : Other antibodies combine with theantigens to form precipitates that are easily ingested by the phagocytes. They are known as precipitins.

(d) Opsonization. Some antibodies coat the surface of the microbes and make them more susceptible to phagocytosis. Such antibodies are known as opsonins.

 Bacteriolysins. These antibodies make the cell wall of bacteria perforated.

Read more Note /--👇🏻 

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