Different classes of Antibodies
There are five classes of antibodies named according to the heavy chain they possess. They are:
- Immunoglobulin G (IgG)
- Immunoglobulin A (IgA)
- Immunoglobulin M (IgM)
- Immunoglobulin E (IgE)
- Immunoglobulin D (IgD)
Immunoglobulin G (IgG)
It is monomeric antibody with molecular weight of 150,000 Da. It has a half-life of 23 days, highest among the immunoglobulins. It has sedimentation coefficient of 7s.
Most abundant class of immunoglobulins in the serum, comprising about 80% of the total serum immunoglobulin. There are four IgG subclasses IgG1, IgG2, IgG3, and IgG4. IgG1, IgG3, and IgG4 are special because these are the only immunoglobulins with the ability to cross the placental barrier.
It consists of one variable VH domain and a constant (C) region with three domains designated CH1, CH2, and CH3.
- Mediate opsonization due to affinity towards Fc receptors on phagocytic cells (IgG1 and IgG3 bind with high affinity, IgG4 with intermediate affinity and IgG2 with extremely low affinity).
- Binds microorganisms and facilitates the process of phagocytosis of microorganisms.
- Takes part in precipitation, complement fixation, and neutralization of toxins and viruses.
- Confers protection against the microorganisms that are present in the blood and tissues.
- Protects the developing fetus against infections.
- Activates complement (IgG3, IgG1, and IgG2, in order of their efficiency).
Immunoglobulin A (IgA)
This dimeric immunoglobulin, comprises nearly 10–15% of serum immunoglobulin. It has a half-life of 6–8 days with sedimentation coefficient 7s.
Predominant immunoglobulin in colostrum, saliva, tears, bronchial secretions, nasal mucosa, prostatic fluid, vaginal secretions, and mucous secretions of the small intestine.
It is of two forms: Serum IgA and Secretory IgA
Serum IgA: present in the serum and is a monomeric 7S molecule with a molecular weight of 60,000 Da. It has a half-life of 6–8 days. It has two subclasses, IgA1 and IgA2, which are two -chain isotypes α-1 and α -2, respectively.
Secretory IgA: dimer or tetramer and consists of a J-chain polypeptide and a polypeptide chain called secretory component, or SC, which is a polypeptide with a molecular weight of 70,000 Da and is produced by epithelial cells of mucous membranes.
Functions of IgA:
- Secretory IgA can play an important first line of defense in antigen clearance by binding to antigens that leak across an epithelium and transporting them back across to prevent their entry.
- Activates alternative pathway of complement.
- Blocks uptake of antigen, bacterial or viral attachment.
- Limits inflammation induced by classical pathway complement activation.
- Promote microbial destruction through ADCC by binding to leukocyte receptors.
Immunoglobulin M (IgM)
IgM is basically a pentamer, composed of five immunoglobulin subunits (monomeric subunits, IgMs) and one molecule of J chain. Two subclasses of IgM (IgM1 and IgM2) are found, which differ in their µ chains. IgM1 consists of µ1 and IgM2 consists of µ2 chains.
IgM antibody constitutes about 5–8% of total serum immunoglobulins and distributed mainly intravascularly. It is a heavy molecule with a molecular weight varying from 900,000 to 1,000,000 Da. It has a half-life of 5 days and has sedimentation coefficient of 19S.
Functions of IgM:
- Highly efficient in activating the classical complement pathway.
- Major immunoglobulin that is expressed on the surface of B cells where it serves as an antigen receptor.
- Gives primary immune response.
- Act as agglutinin, opsonin, lysins and complement fixing antibody.
Immunoglobulin E (IgE)
It constitutes less than 1% of the total immunoglobulin, mostly found extravascularly in lining of the respiratory and intestinal tracts. IgE is an 8S molecule with a molecular weight of 190,000 Da and half-life of 2–3 days. Unlike other immunoglobulins that are heat stable, it is heat-labile (easily inactivated at 56°C in 1 hour).
Also known as reaginic antibody that mediates the type I immediate hypersensitivity (atopic) reaction. Mostly found extravascularly in lining of the respiratory and intestinal tracts.
- Important role in protection against parasitic infection.
- Activates type- I hypersensitive reaction (anaphylactic reaction).
- Localized mast-cell degranulation induced by IgE may also release mediators that facilitate a buildup of various cells necessary for anti-parasitic defense.
Immunoglobulin D (IgD)
It is a 7S monomer with a molecular weight of 180,000 Da. The half-life of IgD is only 2–3 days.
Comprises less than 1% (about 0.2%) serum immunoglobulins. Precise function of IgD remains unclear however, some functions are: