Antibody- Introduction, Structure and Classes


Antibodies are essential components of the adaptive immune system, which is the branch of immunity that can recognize and eliminate specific pathogens and foreign substances. The adaptive immune system consists of two types of lymphocytes: B cells and T cells. B cells are responsible for producing and secreting antibodies, while T cells help to activate B cells and other immune cells.

Antibodies are also known as immunoglobulins (Ig), which means immune proteins. They are Y-shaped molecules that consist of four polypeptide chains: two identical heavy chains and two identical light chains. The chains are held together by disulfide bonds and non-covalent interactions. Each antibody has two antigen-binding sites, one at the tip of each arm of the Y. The antigen-binding sites are formed by the variable regions of the heavy and light chains, which can vary in their amino acid sequences among different antibodies. The variable regions are responsible for recognizing and binding to specific antigens, which are molecules that can elicit an immune response. Antigens can be proteins, polysaccharides, lipids, nucleic acids, or other substances that are foreign to the body.

The stem of the Y-shaped antibody is formed by the constant regions of the heavy chains, which do not vary among antibodies of the same class or isotype. The constant regions determine the biological functions of the antibodies, such as activating complement proteins, binding to Fc receptors on immune cells, or crossing biological barriers. There are five main classes or isotypes of antibodies in humans: IgM, IgG, IgA, IgE, and IgD. Each class has different physical and biological properties that suit different roles in immunity.

Antibodies play a crucial role in defending the body against infections and diseases. They can neutralize toxins, prevent viruses from entering cells, opsonize bacteria for phagocytosis, activate complement system for lysis, or trigger antibody-dependent cellular cytotoxicity (ADCC) by natural killer cells. Antibodies can also mediate allergic reactions, autoimmune disorders, and transplant rejection by binding to self-antigens or foreign antigens on host cells.

Antibodies are produced by B cells in response to antigen stimulation. B cells undergo a process of maturation and differentiation in the bone marrow and peripheral lymphoid organs, such as the spleen and lymph nodes. During this process, B cells rearrange their immunoglobulin genes to generate a diverse repertoire of antibodies with different antigen specificities. B cells also undergo somatic hypermutation and class switch recombination to improve their affinity and change their isotype. Some B cells become plasma cells that secrete large amounts of antibodies into the blood and lymph. Other B cells become memory cells that persist in the body and provide long-lasting immunity.

Antibodies are remarkable molecules that can adapt to various challenges posed by pathogens and foreign substances. They are indispensable for the protection and regulation of the immune system. In this article, we will explore the structure and classes of antibodies in more detail.