Subclasses of immunoglobulins

In addition to the major immunoglobulin classes, several Ig subclasses exist in all members of a particular animal species. Antibodies are classified into subclasses based on minor differences in the heavy chain type of each Ig class. In humans there are four subclasses of IgG: IgG1, IgG2, IgG3 and IgG4 (numbered in order of decreasing concentration in serum).

Variance among different subclasses is less than the variance among different classes. For example, IgG1 is more closely related to IgG2, IgG3 and IgG4 than to IgA, IgM, IgD or IgE. Consequently, antibody-binding proteins (e.g., Protein A or Protein G) and most secondary antibodies used in immunodetection methods cross-react with multiple subclasses but usually not multiple classes of Ig.

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

Structure of immunoglobulins

Antibody (or immunoglobulin) molecules are glycoproteins composed of one or more units, each containing four polypeptide chains: two identical heavy chains (H) and two identical light chains (L). The amino terminal ends of the polypeptide chains show considerable variation in amino acid composition and are referred to as the variable (V) regions to distinguish them from the relatively constant (C) regions. Each L chain consists of one variable domain, VL, and one constant domain, CL. The H chains consist of a variable domain, VH, and three constant domains CH1, CH2 and  CH3. Each heavy chain has about twice the number of amino acids and molecular weight (~50,000) as each light chain (~25,000), resulting in a total immunoglobulin monomer molecular weight of approximately 150,000.

Generalized structure of an immunoglobulin (IgG).

Annotated diagram of immunoglobulin structure.

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Heavy and light chains are held together by a combination of non-covalent interactions and covalent interchain disulfide bonds, forming a bilaterally symmetric structure. The V regions of H and L chains comprise the antigen-binding sites of the immunoglobulin (Ig) molecules. Each Ig monomer contains two antigen-binding sites and is said to be bivalent.

The hinge region is the area of the H chains between the first and second C region domains and is held together by disulfide bonds. This flexible hinge (found in IgG, IgA and IgD, but not IgM or IgE) region allows the distance between the two antigen-binding sites to vary.