First described in 1890, antibodies, also known as an immunoglobulins (Ig), are large Y-shaped glycoproteins that are used by the immune system to identify and neutralize foreign antigens, such as bacteria or viruses. Antibodies are able to bind virtually any non-self surface with exquisite specificity and high affinity. These attributes make antibodies not only key to immunity but also indispensable tools for biomedical research, diagnostics and therapy.
Antibodies are classified into isotypes which are defined by individual types of heavy chain constant domains, each isotype being encoded by a separate constant domain gene. Differences in the heavy chain constant domain between isotypes include the number and location of interchain disulfide bonds, the number of attached oligosaccharide moieties, the number of constant domains, and the length of the hinge region. In mammals, antibodies are divided into five isotypes: IgG, IgM, IgA, IgD and IgE. In humans and mice, the Immunoglobulin IgG is further divided into subclasses (e.g. in mouse IgG1, IgG2a, IgG2b, IgG2c and IgG3) based on small differences in the number of disulfide bonds and the length and flexibility of the hinge region. The various antibody isotypes differ in their biological features, structure, target specificity and distribution.
All antibodies contain two F(ab) regions and one Fc region. The F(ab) regions correspond to the two identical arms of the antibody molecule, which contain the complete light chains paired with the variable and first constant domains of the heavy chains. The Fc region corresponds to the constant domains of the heavy chain, excluding the first constant domain. These three regions, often referred to as fragments, can be separated via proteolytic enzyme digestion. Digestion with the enzyme papain cleaves antibodies into three fragments, two F(ab) fragments and one Fc fragment, while the enzyme pepsin does not separate the two F(ab) fragments resulting in one Fc fragment and one F(ab’)2 fragment consisting of both F(ab) fragments joined by disulfide bonds. The F(ab) and F(ab’)2 fragments retain the ability to bind antigen even after digestion making them ideal for certain immunochemical techniques and experimental applications.