Immunoglobulin (IgG) is a globulin that has antibody activity (specific binding to an antigen) or no antibody activity but is chemically similar to an antibody. Immunoglobulin is the secreted product of B lymphocyte proliferation and differentiation into plasma cells stimulated by antigen. Immunoglobulins are ubiquitous in plasma, interstitial fluid, exocrine fluid, and the surface of cytoplasm and membrane of some cells.