Immunoglobulin G (IgG) is a glycoprotein molecule which belongs to the immunoglobulin family of proteins known as antibodies. Immunoglobulins are the key component of humoral immunity. IgG has an approximate molecular weight of about 150kDa and it is composed of four peptide chains: two identical heavy chains (γ) of about 50kDa and two identical light chains (κ) of about 25kDa each. The heavy chains are linked to each other and to the light chain by disulfide bonds. At the N terminus, both the heavy and the light chain contain variable regions (VH and VL) which account for antibody diversity. At the C terminus, both chains contain constant regions (CH and CL) but only CH mediates effector functions. Structurally the IgG molecule may be divided into: (1) the Fragment antigen binding region (Fab) containing the VL, VH, CL and CH2 all of which shape the antigen binding site and (2) the Fragment crystallizable region (Fc) containing CH domains 2 – 4 which stabilize the antibody and bind to the Fc receptor on the surface of macrophages, neutrophils, natural killer cells as well as to complement proteins to mediate therefore physiological effects.

IgG is synthesized and secreted by plasma B cells in response to an immunogen after recognition of specific epitopes on the antigen and it is generated following class switching and maturation of an antibody response, thus providing immune protection. There are four subclasses of IgG in humans (IgG 1, 2, 3, 4) with variable affinity to Fc receptors and complement. The levels of IgG are generally considered to be indicative of an individual’s immune status and are found decrease in conditions such as hypogammaglobulinemia and X-linked agammaglobulinemia. IgG accounts for 75% of the total human protein and can be found in serum, lymphatic fluid, cerebrospinal fluid, colostrum, milk, urine, saliva, tissues, sweat and skin sebum.

IgG2, the second largest part of IgG isotypes in humans, comprises 20-25% of the main subclass and is the prevalent immune response against carbohydrate-/polysaccharide antigens. Among all IgG isotypes, a deficiency in IgG2 is the most common one and associated with recurring airway/respiratory infections caused by encapsulated bacteria such as pneumococci and/or Haemophilus influenza type B.