IgG is the most common type of antibody , comprising about 80% of the body's total. It is equally divided between the blood and interstitial fluid. IgG antibodies represent a large vocabulary of antigen recognition molecules. There are four subgroups, currently labeled with number suffixes ( IgG1 to 4 ). In some mucosal tissues (e.g. mammary glands of ruminants), the IgG1 class of immunoglobulin-producing cells predominate while IgG4 is the major circulating antibody which enters tissues freely and participates in diverse immune events. IgG (as well as IgM ) activates the complement system.