IgG is the major class of immunoglobulin present in the serum. The IgG molecule consists of two L chains and two H chains (H2L2). There are four subclasses of IgG: IgG1, IgG2, IgG3, and IgG4. Each subtype contains a distinct but related H chain and each differs somewhat regarding their biological activities. IgG1 represents 65% of the total IgG.IgG2- 23%, IgG3 -8% and IgG4 - 4%Reference: Jawetz, Melnick, & Adelberg's Medical Microbiology; 27th edition; Chapter 8; Immunology