Which of the following immunoglobulins has the highest mean serum concentration in humans?
IgA
IgD
IgG
IgM
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