Classes of immunoglobulins

The five primary classes of immunoglobulins are IgG, IgM, IgA, IgD and IgE. These are distinguished by the type of heavy chain found in the molecule. IgG molecules have heavy chains known as gamma-chains; IgMs have mu-chains; IgAs have alpha-chains; IgEs have epsilon-chains; and IgDs have delta-chains.

Differences in heavy chain polypeptides allow these immunoglobulins to function in different types of immune responses and at particular stages of the immune response. The polypeptide protein sequences responsible for these differences are found primarily in the Fc fragment. While there are five different types of heavy chains, there are only two main types of light chains: kappa (κ) and lambda (λ).

Antibody classes differ in valency as a result of different numbers of Y-like units (monomers) that join to form the complete protein. For example, in humans, functioning IgM antibodies have five Y-shaped units (pentamer) containing a total of 10 light chains, 10 heavy chains and 10 antigen-binding.

IgG class

Properties of IgG:

• Molecular weight: 150,000
• H-chain type (MW): gamma (53,000)
• Serum concentration: 10 to 16 mg/mL
• Percent of total immunoglobulin: 75%
• Glycosylation (by weight): 3%
• Distribution: intra- and extravascular
• Function: secondary response

IgM class

Properties of IgM:

• Molecular weight: 900,000
• H-chain type (MW): mu (65,000)
• Serum concentration: 0.5 to 2 mg/mL
• Percent of total immunoglobulin: 10%
• Glycosylation (by weight): 12%
• Distribution: mostly intravascular
• Function: primary response

IgA class

Properties of IgA:

• Molecular weight: 320,000 (secretory)
• H-chain type (MW): alpha (55,000)
• Serum concentration: 1 to 4 mg/mL
• Percent of total immunoglobulin: 15%
• Glycosylation (by weight): 10%
• Distribution: intravascular and secretions
• Function: protect mucus membranes

IgD and IgE class

Properties of IgD:

• Molecular weight: 180,000
• H-chain type (MW): delta (70,000)
• Serum concentration: 0 to 0.4 mg/mL
• Percent of total immunoglobulin: 0.2%
• Glycosylation (by weight): 13%
• Distribution: lymphocyte surface
• Function: unknown

Properties of IgE:

• Molecular weight: 200,000
• H-chain type (MW): epsilon (73,000)
• Serum concentration: 10 to 400 ng/mL
• Percent of total immunoglobulin: 0.002%
• Glycosylation (by weight): 12%
• Distribution: basophils and mast cells in saliva and nasal secretions
• Function: protect against parasites