Immunoglobulin G

Overview

IgG

IgG - B. Catchpole, RVC 2008

Shortened to IgG

IgG is the major antibody in blood plasma, and constitutes at least 80% of all antibodies in the body. It is the smallest immunoglobulin, so can readily leave the blood plasma and enter tissues. They can also cross the placenta, providing adaptive immunity to the foetus when the mother is under attack. IgG is also present in breast milk.

Structure

IgG is Y-shaped, with three constant regions and a heavy chain subunit type γ. There are several different IgG subclasses, depending on the species coded for by the IGHG gene.

Ruminants

Have three subclasses, G1-G3. IgG1 is the major antibody in ruminant mucosal secretions and colostrum.

Dogs, rodents and cats

Have four subclasses. Dogs: G1-G4; Rodents: G1, G2a, G2b, G3. Cats are unknown.

Pigs

Have five subclasses, G1-G4 (G2a, G2b).

Horses

Have six subclasses, G1-G6.

Production

IgG is produced by plasma cells in the spleen, bone marrow and lymph nodes.

Function

Some IgG subclasses can activate complement via the classical pathway. Some subclasses act as targets for macrophages, eosinophils and neutrophils. It is therefore the major antibody in tissue fluids and lymph. IgG specifically binds to antigens on bacteria, causing agglutination and opsonisation.