What are Immunoglobulins?

Antibodies, also known as immunoglobulins (Igs), are Y-shaped glycoproteins critical in the immune system’s defense against infection. Following B cell recognition of a foreign antigen by its B-cell receptor, B cells differentiate into plasma cells that produce and secrete antibodies, which have the same specificity as the receptor. Antibodies bind to antigens with high specificity and affinity, resulting in several effector functions, such as complement activation, enhanced phagocytosis (opsonization), and mast cell activation. Immunoglobulins are indispensable tools for research, diagnostics, and therapies.

Light Chain

Immunoglobulins consist of two identical heavy and two identical light chains. A light chain has an N-terminal variable region and C-terminal constant region. The variable region determines antigen-binding and is created by complex gene rearrangements, followed by somatic hypermutation after antigen exposure, resulting in three regions of sequence hypervariability (complementary-determining regions; CDRs) distributed between four constant sequences (framework regions; FRs). In mammals, there are two types of light chain, λ and κ.

Heavy Chain

Heavy chains also have a variable region for antigen-binding and three or four constant regions that mediate specific effector functions. Heavy chains with three constant regions also have a hinge region, which adds flexibility. There are five types of mammalian heavy chains, μ, γ, α, ε, and δ.

Classes of Immunoglobulins

Antibodies are classified into five isotypes based on these heavy chains as IgM, IgG, IgA, IgE, and IgD, respectively. Each isotype is unique in its structure, localization, and biological functions.


Schroeder HW Jr, Cavacini L. Structure and function of immunoglobulins. J Allergy Clin Immunol. 2010;125(2 Suppl 2):S41-S52. doi:10.1016/j.jaci.2009.09.046

Murphy, K., Weaver, C., & Janeway, C. (2017). Janeway’s immunobiology