Classes of Antibodies

 Classes of antibody

On the basis of their structures and functions, the antibodies are classified into five main classes in humans. They can be memorized as acronym "GAMED".

IgG
IgA
IgM
IgE
IgD

Immunoglobulin Gamma(γ) or  IgG

Immunoglobulin G or Ig G is a monomer having molecular weight of about 146 kDa and serum concentration of 9.0 mg/mL. It is the most abundant class of antibody in the serum. It constitutes about 80 % of the total serum immunoglobulin. It has four subclasses named as IgG1, IgG2, IgG3 and IgG4 their abundance is in decreasing order. 

IgG is mostly synthesized in the secondary immune response. It is the only class of immunoglobulin that is transferred from mother to fetus because it can cross the placenta and is also able to be transferred to the newborn baby from the mother. It is highly protective and activates the classical pathway of the complement system.

Immunoglobulin Alpha(α) or IgA

Immunoglobulin alpha IgA appears in two different molecular structures: monomeric when it is present in the serum and dimeric when it is in the secretory form. The serum IgA is about 160 kDa in molecular weight and serum concentration of 3 mg/ml while the secretory IgA is 385 kDa in molecular weight and has an average serum concentration of 0.05 mg/mL. a J-chain polypeptide joins the two monomers of IgA which then becomes secretory IgA. It also possesses a secretory component of the polypeptide chain that is derived from the receptor responsible for transporting IgA across all cell membranes.It also prevents the IgA from enzymatic digestion. It is the predominant immunoglobulin class in external secretions such as breast milk, saliva, tears, and mucus of the bronchial, genitourinary, and digestive tracts. It activates the alternative pathway of the complement system.

1. Immunoglobulin Mu(µ) or IgM

Immunoglobulin mu or IgM has a molecular weight of about 970 kDa and a mean serum concentration of 1.5 mg/mL. It is the first immunoglobulin that is produced during primary response to antigen means whenever an antigen enters the body, the first immunoglobulin that is produced, is IgM. It may be a monomer that appears as a B cell receptor or a polymer of five monomeric units so the five IgM molecules join together to form a pentameric structure. The monomers of Ig M are joined together by the Fc ends in the center, held together by disulfide bonds and a special joining chain. Because of its pentameric structure, IgM has 10 antigen-binding sites and it has higher valency as compared to the other isotypes. IgM produced by B cells is pentameric while IgM expressed on B cells as B cell receptors is monomeric. IgM stays in the blood where it agglutinates bacteria and enhances the ingestion of pathogens by phagocytic cells. It activates the complement system by the classical pathways. It is also more efficient in activating than IgG for activating the complement system.

Immunoglobulin Epsilon (ε) or IgE

Immunoglobulin epsilon (ε), or IgE is a monomer having a molecular weight of about 188 kDa and a mean serum concentration of 0.3 µg/mL. IgE antibodies are involved in the allergic (hypersensitivity) reactions that are responsible for the symptoms of hay fever, asthma, hives, and anaphylactic shock. IgE has receptors on the membranes of blood basophils and tissue mast cells. Cross-linkage of receptor-bound IgE molecules by antigen (allergen) induces basophils and mast cells to release the contents of their granules to the extracellular environment. As a result, a variety of pharmacologically active mediators are released and give rise to allergic symptoms.

Immunoglobulin Delta (δ) or IgD

Immunoglobulin delta (δ), or IgD is also a monomer. it has a molecular weight of about 164 kDa and a serum concentration of 30µg/ml. It constitutes about 0.2% of the total immunoglobulin in serum.
IgDs are part of the B-cell receptor (BCR) complex. Therefore, their function is to signal the B cell to start antibody production upon initial antigen binding.