Antibody structure and isotypes
Related
Guide to the structural components that make up an antibody - heavy chains, light chains, F(ab)/Fc regions - and antibody isotypes.
Updated May 9, 2022.
Antibody structure
Antibodies, also known as immunoglobulins (Ig), are large, Y-shaped glycoproteins produced by B-cells as a primary immune defense. Antibodies specifically bind unique molecules of a pathogen, called antigens.
Antibodies exist as one or more copies of a Y-shaped unit composed of four polypeptide chains (Fig. 1). Each Y unit contains two identical copies of a heavy chain (H) and two identical copies of a light chain (L); heavy and light chains differ in their sequence and length. The top of the Y shape contains the variable region (V), also known as the fragment antigen-binding (F(ab)) region. This region binds tightly to a specific part of an antigen called an epitope.
The antibody base consists of constant domains (C) and forms the fragment crystallizable region (Fc). This region is essential for the function of the antibody during an immune response.
Figure 1. Antibody structure. The Y-shaped antibody is joined in the middle by a flexible hinge region. Antigen binding occurs at the variable domain (V), consisting of immunoglobulin heavy (H) and light chains (L). The base of the antibody includes constant domains (C). VH – heavy chain variable domain, VL- light chain variable domain, CH – heavy chain constant domain, CL – light chain constant domain.
F(ab) and Fc regions
The Y-shape of an antibody can be cleaved into three fragments by the proteolytic enzyme pepsin: two F(ab) regions and an Fc region. The F(ab) regions contain the variable domain that binds to cognate (specific) antigens. The Fc fragment provides a binding site for endogenous Fc receptors on the surface of lymphocytes and secondary antibodies. Also, dye and enzymes can be covalently linked to antibodies on the Fc portion of the antibody for experimental visualization.
Antibody fragments have distinct advantages in specific immunochemical techniques. Fragmenting IgG antibodies is sometimes useful because F(ab) fragments (1) will not precipitate the antigen, and (2) will not be bound by immune cells in live studies because of the lack of an Fc region. Often, because of their smaller size and lack of cross-linking (due to the Fc region's loss), F(ab) fragments are radiolabeled in functional studies. Fc fragments are often used as Fc receptor blocking agents in immunohistochemical staining.
Learn more about the advantages of F(ab) and F(ab')2 fragments.
Heavy chains
The type of heavy chain defines the overall class or isotype of an antibody. There are five types of mammalian Ig heavy chains denoted by Greek letters: α, δ, ε, γ and μ. These chains are found in IgA, IgD, IgE, IgG, and IgM antibodies, respectively. Heavy chains differ in size and composition; α and γ contain approximately 450 amino acids, while μ and ε have about 550 amino acids.
Each heavy chain has two regions: constant (CH) and variable (VH). The constant region is identical in all the same isotype antibodies but differs in antibodies of different isotypes. Heavy chains γ, α, and δ have a constant region composed of three tandem Ig domains – CH1, CH2, CH3 – and a hinge region for added flexibility. Heavy chains μ and ε have a constant region composed of four immunoglobulin domains. The heavy chain's variable region (VH) differs depending on the B cell that produced it but is the same for all antibodies produced by a single B cell or B cell clone. Each heavy chain's variable region is approximately 110 amino acids long and composed of a single Ig domain.
Light chains
Mammals have only two types of light chains, lambda (λ) and kappa (κ), which have minor differences in the polypeptide sequence. A light chain has two successive domains: constant (CL) and variable (VL). The approximate length of a light chain is 211–217 amino acids. Each antibody contains two light chains that are always identical. Other types of light chains, such as the iota (ι) chain, are found in lower vertebrates like Chondrichthyes and Teleostei.
Antibody isotypes
In mammals, antibodies are divided into five isotypes: IgG, IgM, IgA, IgD, and IgE. Each isotype has a unique structure, as depicted in Figure 2. The isotypes vary based on the number of Y units and the type of heavy chain. They will also differ in their biological properties, functional locations, and the ability to deal with different antigens (Table 1).
Figure 2. Antibody structure and isotypes.
| Isotype | Heavy chain | Light chain | MW (kDa) | Structure | Function |
IgA1 IgA2 | α1 α2 | λ or κ | 150–600 | Monomer - tetramer | Most abundantly produced antibody isotype in mice and humans. It is 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, triggers histamine release from mast cells, and is involved in allergy. It also protects against parasitic worms. |
| IgG1 IgG2a IgG2b IgG3 IgG4 | γ1, γ2, γ3, γ4 | λ or κ | 150 | Monomer | The most abundant Ig in serum that accounts for ~75% of the total serum antibodies in humans. Provides the majority of antibody-based immunity against invading pathogens. Moderate complement fixer. |
| 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. |
Antigen-antibody interactions: how and where antibodies bind
The F(ab) antibody region contains the antigen-binding site called paratope. The paratope binds to a specific part of an antigen called the epitope, which is a small part of the antigen – sometimes just a few amino acids long (Fig. 3).
The paratope and epitope are held together by complementary shapes and intermolecular interactions such as Van der Waals, hydrogen bonds, electrostatic and hydrophobic interactions; the strength of these forces determines the antibody's affinity.
Figure 3. A schematic representation of antigen-antibody interactions.