Antibody Forms

Five classes of antibody

Antibodies or immunoglobulins come in a variety of forms. Based on differences in the amino acid sequences at the constant region of the heavy chains they are further classified into five classes. These are:

• IgG - containing gamma heavy chain
• IgM – containing mu heavy chain
• IgA - containing alpha heavy chain
• IgD – containing delta heavy chain
• IgE – containing epsilon heavy chain

Subclasses of antibodies

Each of the forms has a small difference in the constant region of the heavy chain. Based on the differences the Igs are classified into subclasses. These are detected by serological means.

The subclasses include:

• IgG1 - Gamma 1  heavy chains
• IgG2 - Gamma 2  heavy chains
• IgG3 - Gamma 3  heavy chains
• IgG4 - Gamma 4  heavy chains
• IgA Subclasses
  • IgA1 - Alpha 1  heavy chains
  • IgA2 - Alpha 2  heavy chains

The Immunoglobulins are further classified by the type of light chain that they have. Light chain types are based on differences in the amino acid sequence in the constant region of the light chain. There are two types of light chains – Kappa and Lambda side chains.
 

Based on the light chains there are further subtypes. For example the Lambda subtypes include:

a) Lambda 1 
b) Lambda 2 
c) Lambda 3 
d) Lambda 4 

IgG

These are monomeric structures that exist as single molecules. These are the most versatile immunoglobulins and can carry out all functions of Ig molecules. This forms the largest portion in the serum and is also found in extravascular spaces. This is the only immunoglobulin that crosses the placenta. It also fixes molecules called complements.  It binds to cells and enhances phagocytosis.

IgA

These are also monomeric structures. They are found in secretions as a dimer having a J chain. IgA can move across mucosa without degradation. It is the second most abundant Ig in serum. It is the major class of Ig in secretions i.e. in tears, saliva, colostrum (initial breast milk), mucus etc. and is important in mucosal immunity. It binds to PMN cells and lymphocytes. It does not normally fix complement.

IgM

These have an extra domain on the mu chain (CH4) and another protein covalently bound via S-S. These exist is J shapes as polymers. Usually they form pentamers or clusters of 5. It is the first Ig to be made by fetus. It is the third most abundant Ig in serum. It fixes with complements and is a good agglutinating Ig that leads to elimination of microbes. It is also able to bind some cells via Fc receptors.

IgD

These exist as monomers. They have low serum levels. It is found primarily on B cells surface and serves as a receptor for antigens. It does not fix complement.

IgE

These occur as monomers. This is the least common Ig in serum. They bind very tightly to Fc receptors on basophils and mast cells before interacting with antigens. Thus they are involved in allergic reactions. It plays a role in parasitic helminthic diseases.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)