Intravenous immunoglobulin (IVIG) is a preparation of human immunoglobulin pooled from thousands of healthy individuals. It was originally developed for replacement therapy in humoral immunodeficiency syndromes but has more recently become an important theurapetic modality in severe autoimmune disorders, such as thrombocytopenic purpura, AIHA, neuroimmunological diseases such as Guilland Barre syndrome, SLE, certain forms of vasculitis, and polymyositis/dermatomyositis. The mechanism of action remains unclear, but IVIG may block the function of Fc receptors expressed by phagocytes of the reticuloendothelial system and also induces FcγRIIB (inhibitory Fc receptor) expression on infiltrating macrophages in the K/BxN model of RA. An additional mode of action may involve the presence of anti-idiotypic antibodies that block the antigen combining sites of pathogenic antibodies. The duration of action is limited by the metabolism of serum immunoglobulin, and generally, IVIG is regarded as a temporary measure that is followed by modified tharapy.