Glycoproteins, termed antibodies, are produced by B lymphocytes in a membrane-bound or secreted state responding to exposure to foreign elements called antigens. A family of proteins — human immunoglobulins — promotes humoral immunity and plays an essential function in establishing cellular immunity.

The five major classes of human immunoglobulins are immunoglobulin G (IgG), IgA, IgM, IgD, and IgE. These classes share the same fundamental four-chain structured antibody but vary in their heavy chains, γ, μ, α, δ and ε. There are only two primary forms of light chains: kappa (κ) and lambda (λ).

Properties of IgG

Immunoglobulin G (IgG) represents the most prevalent of all immunoglobulins (75%).
IgG1, IgG2, IgG3, and IgG4 are the four subclasses of IgG.
It protects the growing foetus against infections.
Due to their affinity for Fc receptors on phagocytic cells, they mediate opsonisation.
It enhances complement (IgG3, IgG1, and IgG2, in order of their efficiency).
Attracts microorganisms and promotes phagocytosis.
Protects against the pathogens found in tissues and blood.
Participates in complement fixation, neutralisation of toxins, and precipitation.

Properties of IgA

The second most abundant Ig is immunoglobulin A (IgA), which has two subclasses: IgA1 and IgA2.
IgA is primarily found in tears, saliva, bronchial secretions, prostatic fluid, oral mucosa, vaginal secretions, colostrum, small intestine secretions, and breast milk.
Its molecular weight is 400,000 Da, comprising 15% of the total Ig.
It reduces the inflammation produced by complement activation in the classical pathway.
It prevents viral, bacterial, or antigen attachment uptake.
There are two 4-chain basic units in the secretory IgA molecule. A short peptide connects the J-chain.
By attaching to leukocyte receptors, they facilitate microbial elimination via ADCC (Antibody-dependent cellular cytotoxicity).
It activates the alternative complement pathway.

Properties of IgM

A pentamer, immunoglobulin M (IgM) comprises five monomeric immunoglobulin subunits (IgMs) and one J-chain molecule.
There are two IgM subclasses (IgM1 and IgM2) that their µ chains can distinguish. IgM1 and IgM2 are composed of µ1 and µ2 chains, respectively.
Approximately 10% of immunoglobulins are IgM. Due to its large size, it is primarily contained inside the intravascular space.
IgM monomeric form is a B-cell antigen receptor. The heavy chain has four domains.
IgM is very effective in stimulating the classical complement pathway.
It is a primary immunoglobulin which functions as an antigen receptor and is found on the membrane of B cells.
It produces a primary immunological response.

Properties of IgD

Blood contains minimal amounts of immunoglobulin D (IgD).
It comprises 0.2% of the total immunoglobulin, 3-5 mg/dl.
It coexists with IgM on the surface of B lymphocytes. IgD and IgM act as antigen recognition receptors.
IgD is composed of four heavy chain domains.
Although its primary function is unknown, it may be involved in B-lymphocyte differentiation.
It might contribute to lymphocyte differentiation brought on by antigens.
It is unable to pass through the placental wall.

Properties of IgE

IgE, or immunoglobulin E, is an antibody that induces type-I hypersensitivity reaction (allergic reaction).
It is primarily located extravascularly in the lining of the digestive and respiratory tracts.
IgE is a monomer that composes 0.004% of all immunoglobulin and has a molecular weight of 190,000 Da.
Due to the four CH domains, it is the largest Ig monomer.
It cannot activate complement through the classical pathway.
It cannot pass the placental barrier.

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Frequently Asked Questions – FAQs

What are the five human immunoglobulins?

IgG, IgA, IgM, IgD, and IgE are the five main types of immunoglobulins. These can be identified by the form of heavy chains present in the molecule. IgM molecules have mu-chains, IgA molecules have alpha-chains, IgE molecules have epsilon-chains, IgD molecules have delta-chains, and IgGs have heavy gamma chains.

What is the function of immunoglobulins in the human body?

Antibodies, also known as immunoglobulins, are glycoprotein molecules synthesised by white blood cells or plasma cells. They play an essential role in the immune response by selectively identifying and attaching to specific antigens, like viruses or bacteria, and helping their elimination.

What is an immunoglobulin G (IgG) deficiency?

Immunoglobulins or antibodies are unique proteins our body produces when it senses an attack. The insufficient production of immunoglobulin G (IgG) by our body is known as an IgG deficiency. Infection risk is higher in people with IgG deficiencies.

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