Antibodies - Immunoglobulin

Immunoglobulins (Ig) also known as antibodies, are a critical component of the adaptive immune system. Produced by plasma cells (activated B cells), they are glycoproteins that specifically recognize and bind to antigens, such as pathogens or foreign substances, to neutralize or mark them for destruction by other immune cells. There are five main classes of immunoglobulins in humans, each with unique functions, structures, and locations in the body. 

Classification of Immunoglobulin 

 1. Immunoglobulin G (IgG)

- Structure: IgG is a monomer, consisting of two heavy chains and two light chains, forming a Y-shaped molecule.

- Subclasses: There are four subclasses in humans: IgG1, IgG2, IgG3, and IgG4, each differing in their hinge region length and flexibility, which influences their ability to bind antigens and activate complement.

- Function: IgG is the most abundant antibody in the serum, accounting for about 75-80% of all immunoglobulins. It plays a major role in secondary immune responses and long-term immunity. IgG can neutralize toxins and viruses, opsonize bacteria (marking them for phagocytosis), and activate the classical complement pathway.

- Special Features: IgG is the only antibody class that can cross the placenta, providing passive immunity to the fetus. It has a long half-life, around 23 days, which helps maintain long-term immunity.

2. Immunoglobulin A (IgA)

- Structure: IgA primarily exists as a dimer (two Y-shaped units linked by a J-chain) in mucosal areas but can also be found as a monomer in the blood.

- Subclasses: There are two subclasses: IgA1 and IgA2. IgA1 is predominant in serum, while IgA2 is more resistant to bacterial proteases and is found in mucosal secretions.

- Function: IgA is the main immunoglobulin found in mucosal areas, such as the gastrointestinal, respiratory, and urogenital tracts, and in secretions like saliva, tears, and breast milk. It plays a crucial role in mucosal immunity by neutralizing pathogens and preventing their adherence to epithelial cells. In breast milk, IgA provides passive immunity to infants, protecting their gastrointestinal tract from infections.

- Special Features: IgA has a secretory component that protects it from degradation by proteolytic enzymes in mucosal secretions, enhancing its stability and function in harsh environments.

 3. Immunoglobulin M (IgM)

- Structure: IgM is a pentamer in its secreted form (five Y-shaped units joined by a J-chain) and a monomer when membrane-bound on B cells.

- Function: IgM is the first antibody produced during the primary immune response to an antigen and is very effective at forming complexes with antigens and activating the complement system via the classical pathway. Due to its pentameric structure, IgM has a high avidity for antigens, meaning it can bind multiple epitopes simultaneously, making it effective at agglutinating pathogens and enhancing their clearance.

- Special Features: IgM is primarily found in the blood and lymphatic fluid due to its large size, which restricts its ability to diffuse into tissues. It also plays a critical role in early-stage immune responses before sufficient levels of IgG are produced.

 

4. Immunoglobulin E (IgE)

- Structure: IgE is a monomer similar in structure to IgG but with an additional constant heavy chain domain.

- Function: IgE is involved in the immune response against parasitic infections, such as helminths. It binds to high-affinity IgE receptors (FcεRI) on mast cells and basophils. Upon binding to an antigen (allergen), IgE cross-linking triggers these cells to release histamine and other mediators, leading to allergic reactions, including symptoms such as bronchoconstriction, vasodilation, and increased mucus production.

- Special Features: IgE has the shortest half-life in serum (2–3 days) but, when bound to its receptor on mast cells or basophils, can remain for weeks or months. It is present in low concentrations in the serum under normal conditions but can increase significantly in allergic individuals or during parasitic infections.

5. Immunoglobulin D (IgD)

- Structure : IgD is a monomer and is structurally similar to IgG, with a long hinge region that provides flexibility.

- Function : The function of IgD is not entirely understood. It is found in small amounts in the serum and is primarily expressed on the surface of immature and mature B cells. It is thought to play a role in the activation and regulation of B cells during the early stages of the immune response, possibly in initiating antigen-specific B cell activation.

- Special Features : IgD is co-expressed with IgM on the surface of naive B cells, suggesting a role in B cell development and function. Unlike other immunoglobulins, IgD does not seem to participate significantly in complement activation or have a direct role in pathogen neutralization.