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Five Classes Of Immunoglobulin


Five Classes Of Immunoglobulin
Antibodies are often referred to as immunoglobulins (immune proteins). There are five classes of antibodies or immunoglobulins, termed immunoglobulin G (IgG), IgM, IgA, IgD, and IgE. All these classes have the basic four‐chain antibody structure but they differ in their heavy chains, which are termed γ, μ, α, δ, and ε, respectively. 

The differences are most pronounced in the Fc regions of the antibody classes and this leads to the triggering of different effector functions on binding to antigen. For example, IgM recognition of antigen might lead to complement activation, whereas IgE recognition (possibly of the same antigen) might lead to mast cell degranulation and anaphylaxis (increased vascular permeability and smooth muscle contraction). These differences are discussed in greater detail later. Structural differences also lead to differences in the polymerization state of the monomer unit shown in Figure 3.1. Thus, IgG and IgE are generally monomeric, whereas IgM occurs as a pentamer. IgA occurs predominantly as a monomer in serum and as a dimer in seromucous secretions.
The antibody basic unit (IgG is represented), consisting of two identical heavy and two identical light chains held together by interchain disulfide bonds (a), can be broken down into its constituent polypeptide chains and to proteolytic fragments, the pepsin F(ab′)2 retaining two binding sites for antigen (c) and the papain Fab with one (b). After pepsin digestion the pFc′ fragment representing the C‐terminal half of the Fc region is formed and is held together by noncovalent bonds. The portion of the heavy chain in the Fab fragment is given the symbol Fd. The N‐terminal residue is on the left for each chain.

The major antibody in the serum is IgG and, as this is the best‐understood antibody in terms of structure and function, we shall consider it first. The other antibody classes will be considered in relation to IgG.

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