Antibodies, also known as immunoglobulins, are a variety of glycoproteins that B lymphocytes produce in response to foreign elements, known as antigens. These human immunoglobulins are critical for promoting humoral immunity and play a pivotal role in cellular immunity.

The human body produces five major classes of immunoglobulins: Immunoglobulin G (IgG), IgA, IgM, IgD, and IgE. While they all share the basic four-chain structure of an antibody, they differ in their heavy chains, γ, μ, α, δ and ε. The two main types of light chains are kappa (κ) and lambda (λ).

Table of Contents

• Characteristics of IgG
• Characteristics of IgA
• Characteristics of IgM
• Characteristics of IgD
• Characteristics of IgE

Characteristics of IgG

• IgG is the most common type of immunoglobulins, accounting for 75% of them all.
• There are four subclasses of IgG: IgG1, IgG2, IgG3, and IgG4.
• This antibody plays a crucial role in protecting the fetus from infections.
• They contribute to opsonisation due to their affinity for Fc receptors on phagocytic cells.
• They stimulate the complement system, particularly IgG3, IgG1, and IgG2.
• They attract microorganisms and facilitate phagocytosis.
• IgG antibodies defend against pathogens present in tissues and blood.
• They are involved in complement fixation, toxin neutralisation, and precipitation.

Characteristics of IgA

• Immunoglobulin A (IgA) is the second most common type of Ig, with two subclasses: IgA1 and IgA2.
• IgA is predominantly found in tears, saliva, bronchial secretions, prostatic fluid, oral mucosa, vaginal secretions, colostrum, small intestine secretions, and breast milk.
• It has a molecular weight of 400,000 Da, making up 15% of the total Ig.
• It helps to reduce inflammation caused by the activation of the classical complement pathway.
• It prevents the attachment and uptake of viruses, bacteria, or antigens.
• Secretory IgA molecules consist of two 4-chain basic units connected by a short peptide known as the J-chain.
• They aid in the elimination of microbes via ADCC (Antibody-dependent cellular cytotoxicity) by binding to leukocyte receptors.
• It triggers the alternative complement pathway.

Characteristics of IgM

• Immunoglobulin M (IgM) is a pentamer made up of five monomeric IgM subunits and a J-chain molecule.
• There are two subclasses of IgM (IgM1 and IgM2), distinguished by their µ chains. IgM1 and IgM2 consist of µ1 and µ2 chains, respectively.
• IgM makes up approximately 10% of immunoglobulins and is primarily found in the intravascular space due to its large size.
• The monomeric form of IgM serves as a B-cell antigen receptor. The heavy chain has four domains.
• IgM is particularly potent in activating the classical complement pathway.
• It's a primary immunoglobulin that works as an antigen receptor and is located on the membrane of B cells.
• It is responsible for generating a primary immunological response.

Characteristics of IgD

• Minimal amounts of immunoglobulin D (IgD) are found in the blood.
• It makes up 0.2% of the total immunoglobulin, 3-5 mg/dl.
• It coexists with IgM on the surface of B lymphocytes. Both IgD and IgM function as antigen recognition receptors.
• IgD consists of four heavy chain domains.
• While its primary function remains unclear, it is believed to play a role in B-lymphocyte differentiation.
• It may contribute to lymphocyte differentiation induced by antigens.
• It cannot pass through the placental barrier.

Characteristics of IgE

• IgE, or immunoglobulin E, is an antibody responsible for triggering type-I hypersensitivity reactions (allergic reactions).
• It is primarily found in the linings of the digestive and respiratory tracts.
• IgE is a monomer that constitutes 0.004% of all immunoglobulins and has a molecular weight of 190,000 Da.
• It is the largest Ig monomer due to its four CH domains.
• It is unable to activate the complement system through the classical pathway.
• It cannot cross the placental barrier.