ABSTRACT
Immunoglobulins are complex proteins composed of units constituted by two heavy chains and two light chains. There are five isotypes of immunoglobulins. IgG, IgD, and IgE are formed by a single unit, while circulating IgM is formed by five units, and IgA is polymorphic. Both heavy and light chains have constant and variable regions, and the variable regions form the antibody-binding site. IgG and IgA have different subclasses. The IgG molecule can be split with proteolytic enzymes in an Fc fragment (or fragments) and two Fab fragments or one F ' ( a b 2 ' ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429278990/ab21b9d9-381a-450e-a4ab-cab8ce95d393/content/inline-math5_1.jpg"/> fragment, both containing the antigen-binding site but lacking the ability to activate the complement system or to bind to Fcγ receptors. The chapter further discusses the structural and biological differences between the classes and subclasses of immunoglobulins with significant implications in clinical medicine.
