Final answer:
Antibodies are classified into five major classes based on the structure of their heavy chains: IgM, IgG, IgA, IgD, and IgE. The variable domain determines the specificity, while the class or isotype of the heavy chain dictates its immune function.
Step-by-step explanation:
The heavy chain classes of antibodies are vital for determining their specific immune functions. Antigens bind to a heavy chain through its variable domain (VH), which differs across antibodies, while the constant domain (CH) determines the class or isotype of the antibody, influencing the antibody's role in the immune response. There are five main antibody classes differentiated by their heavy chains: IgM, IgG, IgA, IgD, and IgE. These classes have unique heavy chains denoted by the Greek letters μ, γ, α, δ, and ε, respectively. IgG antibodies, which are the most abundant, have heavy chains with one variable domain and three constant domains. IgA and IgD have a similar structure with three constant domains, whereas IgM and IgE contain four constant domains in their heavy chains. Each heavy chain class imparts different effector functions, influencing how the antibody will interact with other elements of the immune system.
In addition to these classes, there are also subclasses within these classes, such as IgG1, IgG2, IgG3, and IgG4, which differ by the number of disulphide bonds and some other structural features. Each antibody class and subclass plays a role in a particular aspect of the immune response, such as activating complement proteins or facilitating opsonization.