What are the subclasses of immunoglobulins?

Immunoglobulins are divided into 5 classes - IgG, IgM, IgA, IgD, and IgE. Of these five classes, IgG is the most abundant in human serum and can be further divided into 4 subclasses - IgG1, IgG2, IgG3, and IgG4, listed in order of decreasing abundance in human serum. All IgG subclasses occur in the serum in monomeric form.