How many classes of antibodies exist in humans based on the heavy chain? 

Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as ${\mathrm{H}}_2{\mathrm{L}}_2$. There are five types immunoglobulin heavy chains: γ, δ, α, μ and ε. They define classes of immunoglobulins: IgG, IgD, IgA, IgM and IgE, respectively.