Which of the following immunoglobulins has the highest mean serum concentration in humans?
IgG is the most abundant class of immunoglobulins in the serum, comprising about 80% of the total serum immunoglobulin.
There are four IgG subclasses IgG1, IgG2, IgG3, and IgG4—so numbered according to their decreasing concentrations in serum.

Parija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:96
IgA
IgD
IgG
IgM