There are 5 major antibody isotypes - IgM, IgD, IgG, IgE, and IgA - which differ based on their heavy chain. The heavy chain determines the isotype and can be mu, delta, gamma, epsilon, or alpha. Light chains can be either kappa or lambda with any isotype. IgG is the most abundant in humans while IgE is the least. Isotypes are located in the constant region of the heavy and light chains. Allotypes are specified by allelic forms of immunoglobulin genes and are also in the constant regions. Idiotypes are unique epitopes located in the variable regions of individual antibody molecules.

1.  Isotypes:
 Antibody isotypes are the same thing as antibody classes.
 There are 5 major isotypes: IgM, IgD, IgG, IgE, and IgA.
 The difference between these isotypes lies in the heavy chain
(Mu, Delta, Gamma, Epsilon, or Alpha).
 You can have either kappa or lambda light chains with any of
these isotypes.
 In humans, the most plentiful isotype is IgG;
 the least plentiful one is IgE.
 Location: Constant region of heavy chain and light chain.
 Allotype:
 Allotype antigenic determinants are specified by the allelic
forms of the Ig genes.
 Location : Constant region of heavy chain and light chain

 Idiotypes:
 Idiotypes are antibodies that recognize different specific
epitopes.
 Idiotypes are the unique antigenic determinants present on
variable heavy chain and variable light region of individual
antibody molecules.
 Location: Variable region of heavy chain and light chain.