Final answer:
Antibodies are classified by the type of heavy chain, which determines the immunoglobulin class or isotype, such as IgG, IgM, IgA, IgD, or IgE. The heavy chains, paired with light chains, form the variable regions that specify antigen binding, while constant regions define the antibody's class and function.
Step-by-step explanation:
Antibodies are classified not only by their function, but by the type of heavy chain. Antibodies, also known as immunoglobulins, are glycoproteins that have two types of polypeptide chains: the heavy chain and the light chain. The constant region of an antibody molecule determines its class, or isotype, with each class characterized by unique heavy chains denoted by Greek letters (γ for IgG, μ for IgM, α for IgA, δ for IgD, and ε for IgE).
The variable region of the heavy and light chains forms the antigen-binding sites, allowing antibodies to bind to specific epitopes on antigens. The structural differences of the heavy chain constant domains between the classes result in distinct immunological roles and mechanisms of action. The heavy chains have one variable domain and three or four constant domains, depending on the antibody class, and interact with the light chain variable domains to create the binding specificity of each antibody.