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COVID-19: Immunology, vaccines and epidemiology
COVID-19: Immunology, vaccines and epidemiology

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3.1 Antibody classes

Secreted IgG is produced as a monomer of the basic antibody structure (2 heavy chains, 2 light chains) but IgM is a pentamer and IgA is a dimer, as illustrated in Figure 9 and 10 below.

Diagram of an IgG molecule
Figure 9 IgG molecule.
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In the Y-shaped antibody molecule, each heavy chain comprises a sequence of three constant domains, CH1, CH2 and CH3, are linked to a single variable domain (VH) at the end of each prong of the Y. Each light chain comprises one C-domain (CL) linked to a single V-domain (VL), both aligned with the corresponding domains (CH1 and VH) of a heavy chain. The hinge region is at the fork of the Y, and the antigen binding sites are at the ends of each fork and are formed by a heavy and a light chain V-domain (i.e. VH with VL). The prongs of the Y constitute the Fab region of the molecule, while the stem (comprising CH2 and CH3) is termed the Fc region.

Figure 9 IgG molecule.

In Figure 9, the four polypeptide chains of an IgG molecule are folded into domains. The variable (V) domains of the heavy and light chains (VH, VL) form the antigen-binding sites. The remaining domains are relatively constant (C domains). The hinge confers segmental flexibility.

Diagram of (a) IgM pentamer and (b) IgA a dimer.
Figure 10 (a) IgM is a pentamer of the basic four-chain immunoglobulin structure whereas (b) IgA is a dimer.
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Part (a) shows the five IgM units arranged radially. It also shows the domain structure of an IgM molecule. Each heavy chain comprises a sequence of four C-domains (Cµ1, Cµ2, Cµ3 and Cµ4) linked to a single variable domain (VH). And, as for IgG, each light chain comprises one C-domain (CL) linked to a single V-domain (VL). At the centre of the IgM pentamer is the J-chain, which is linked to two adjacent Cµ4 chains via disulfide bonds. Other disulfide bonds in the structure link adjacent Cµ3 domains and others bridge adjacent Cµ4 domains. Either one or two carbohydrate units are present on each of the Cµ domains and there are also two of these on the J-chain. Part (b) shows the linear dimeric structure of human secretory IgA (sIgA). It also shows the domain structure of an IgA molecule. As for IgG, each heavy chain comprises a sequence of four C-domains (here called Cα1, Cα2 and Cα3) linked to a single variable domain (VH); and each light chain comprises one C-domain (CL) linked to a single V-domain (VL). A J-chain links opposing Cα3 chains of each monomer via disulfide bonds. Pairs of disulfide bonds are also present at the hinge region of each monomer. There are single carbohydrate units on some of the Cα chains, and there are two on the J chain as in the IgM pentamer. The secretory component is a 5-domain molecule spanning both the J-chain and the flanking Cα2 and Cα3 chains of each monomer.

Figure 10 (a) IgM is a pentamer of the basic four-chain immunoglobulin structure whereas (b) IgA is a dimer.

In Figure 10, each molecule is joined by an extra joining (J) chain. When IgA is secreted, it has an additional chain, the secretory component. (Note: carbohydrate units are shown in blue and inter-chain disulphide bonds in red.)

  • How many antigen-binding sites does IgM have?

  • Ten

The multiple antigen-binding sites of IgM make it very efficient at cross-linking antigens and therefore useful as the first antibody produced in an immune response. However, the affinity of IgM for its target antigens is lower than the affinity of IgG and IgA antibodies produced later in an immune response.

You will now look at how antibodies can protect against virus infection.