Immunoglobulin G (IgG)- Structure, Subclasses and Functions


Immunoglobulin G (IgG) is one of the five classes of antibodies (also known as immunoglobulins) that are produced by the immune system to fight against foreign invaders such as bacteria, viruses and toxins. Antibodies are proteins that can recognize and bind to specific antigens (molecules that trigger an immune response) on the surface of pathogens or other substances. By binding to antigens, antibodies can neutralize them, mark them for destruction by other immune cells or activate various immune mechanisms.

IgG is the most abundant and versatile antibody class in humans, accounting for about 75% of the total serum immunoglobulins. IgG antibodies are involved in many aspects of the immune defense, such as preventing infections, enhancing phagocytosis (the process of engulfing and destroying microbes by specialized cells), activating the complement system (a group of proteins that enhance the immune response) and crossing the placenta to protect the fetus from infections.

The structure of IgG antibodies is complex and consists of four polypeptide chains: two identical heavy chains and two identical light chains. Each chain has a variable region and a constant region. The variable regions are responsible for binding to specific antigens, while the constant regions determine the antibody class and function. The four chains are held together by disulfide bonds (covalent bonds between sulfur atoms) to form a Y-shaped molecule. The two arms of the Y are called Fab fragments (fragment antigen-binding) and contain the antigen-binding sites. The stem of the Y is called Fc fragment (fragment crystallizable) and mediates various effector functions.

There are four subclasses of IgG in humans: IgG1, IgG2, IgG3 and IgG4. They differ in their heavy chain constant regions, which affect their ability to bind to different receptors on immune cells and activate different immune pathways. Each subclass has its own characteristics and roles in the immune response.

In this article, we will discuss the structure, subclasses and functions of IgG antibodies in more detail. We will also explore some of the clinical applications and implications of IgG antibodies in diagnosis, treatment and prevention of diseases.