Structure and Components of Antibodies

Introduction

This article deals with the structure of an antibody, including the heavy chains, light chains, F(ab)/Fc regions, and the isotypes.

An antibody is chemically a glycoprotein that has affinity to bind certain specific antigens. They are produced by living cells in response to the invasion of the system by foreign molecules. They are Y-shaped in structure and contain four polypeptide units.

The Y-shaped component is comprised of two copies each of a heavy chain and a light chain, these being distinguished by variations both in the amino acid sequence and length of the chain. The Y-shape is the location of the variable region, which is found at the top of the molecule. This region is crucial as it is responsible for the particular affinity of binding tightly to a single epitope on the antigen, which enables the antibody to detect the antigen specifically.

Light chains within an antibody molecule are either kappa (κ) or lambda (λ) in type. This difference is based upon minor sequence changes. The antibodies are classified into several categories or classes by the composition of the heavy chain, as shown in Figure 1.

Antibody Isotypes

Antibodies in mammals are found to occur in five isotypes: IgG, IgM, IgA, IgD and IgE. This division is based upon the number of Y units and heavy chain type. The isotypes show variation in biological behavior, have different functional locations and deal with different antigens using different mechanisms.

Isotype Heavy chain Light chain MW
(kDa)
Structure Function

IgA1
IgA2

α1
α2

λ or κ

150–600
Monomer - tetramer Most produced ​Ig. Found in mucosal areas, such as the gut, respiratory and urogenital tract, and prevents their colonization by pathogens. Resistant to digestion and is secreted in milk.
IgD δ λ or κ 150 Monomer Function unclear. Works with IgM in B cell development; mostly B cell bound
IgE ε
λ or κ
190 Monomer Binds to allergens and triggers histamine release from mast cells and is involved in allergy. Also protects against parasitic worms.
IgG1
IgG2a IgG2b
IgG3
IgG4
γ1, γ2, γ3, γ4
λ or κ
150 Monomer Major Ig in serum. Provides the majority antibody based in immunity against invading pathogens. Moderate complement fixer. IgG3 can cross placenta.
IgM
μ

λ or κ
900 Pentamer First response antibody. Expressed on the surface of B cells and in a secreted form with very high avidity. Eliminates pathogens in the early stages of B cell mediated immunity before there is sufficient IgG.

Heavy Chains

The class of an antibody depends upon the heavy chain type. Thus, the five types of Ig heavy chain  which are denoted by the Greek letters α, δ, ε, γ and μ characterize  IgA, IgD, IgE, IgG and IgM antibodies, respectively. The α and γ heavy chains have about 450 amino acids while the μ and ε heavy chains have 550 amino acids. The composition of the chains is also different.

A heavy chain has two parts, a constant region and a variable region. The constant region does not differ between antibodies which have the same isotype, but does vary between isotypes. The heavy chains of type γ, α and δ contain constant regions comprising three domains of Ig in tandem with a hinge region that gives it flexibility.

On the other hand, heavy chains containing μ and ε have a constant region that has four Ig domains. All antibodies produced by a single B cell or its clones are identical with respect to the variable region. However, antibodies produced by different B cells have different variable regions. This region is composed of a single Ig domain and contains about 110 amino acids.

Light Chains

Mammalian antibodies show two types of light chain, namely, λ and κ. Each light chain consists of a constant domain and a variable domain in succession, which make up about 211-217 amino acids. The two light chains in each antibody molecule are identical. Lower vertebrates, such as Chondrichthyes and Teleostei, have an iota (ι) chain as well.

F(ab) and Fc Regions

An antibody contains three sections within the Y-shape, of which two are F(ab) regions and one is an Fc region. The F(ab) regions are the sites of the variable domain that binds to the specific antigens recognized by the antibody.

The Fc section is the site where endogenous Fc receptors found on the cell surface of lymphocytes bind to the antibody, and is also the place where secondary antibodies bind. Enzymes and dyes also form covalent linkage to the antibodies at this fragment, which is made use of therefore for visualizing bound antibodies during experiments.

The enzyme pepsin cleaves peptide bonds to produce these three regions from the Y-part of the molecule. Such fragments are useful in some immunochemistry applications. For instance, F(ab) fragments of IgG antibodies can be used for binding because they have two advantages: they neither precipitate the bound antigen nor are they taken up by immune cells during in vivo studies, due to the absence of the Fc fragment.

They are often labeled with radioactive substances to make use of them in functional assays, as they are small and fail to show crosslinking as a result of the absence of the Fc region. Fc fragments can also be made use of to block Fc receptors during immunohistochemical staining.

About Abcam

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Last updated: Jul 14, 2018 at 6:31 PM

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