Intravenous immunoglobulin (IVIG) is a safe preparation, made of human plasma of thousands of healthy donors. The fascinating history of gamma globulin therapy begins in 1930 when Finland treated pneumococcal pneumonia patients with equine serum, which prolonged their survival from pneumonia. Since then, significant breakthroughs were achieved by Cohn, Bruton, Imbach, and others, whose clinical contribution to the world of medicine was of great importance. Originally IVIG was used to treat immunodeficiencies. Later on the use of IVIG extended to autoimmune diseases as well. The efficacy of IVIG has been established only in several autoimmune diseases; clinical reports of trials, series, and case reports indicate significant improvement in many more autoimmune diseases. IVIG have also showed antimetastatic effects in a variety of cancer cell lines, as well as in a few case reports. The efficiency of IVIG has also been observed in recurrent pregnancy loss (RPL), either as a result of an autoimmune disease or spontaneous. Several attempts were made to discover the immunomodulatory effects of IVIG, but it is still not fully understood. Clearly IVIG has multiple mechanisms of actions, which are thought to cooperate synergistically. One of the main mechanisms of actions of IVIG is its ability to neutralize pathogenic autoantibodies via anti-idiotypic antibodies within IVIG preparation. The ability of IVIG to neutralize pathogenic autoantibodies is of great importance in many autoimmune diseases, as well as in RPL. In cancer cell lines, IVIG modulates the immune system in a few ways, including the induction of IL-12 secretion, which consequently activates natural killer cells, and the induction of expression of proapoptotic genes only in cancer cells. Side effects from IVIG are rare and mostly mild and transient. More importantly adverse effects can be minimized by administration to a selective patient population in a proper way: slow infusion rate of 0.4 g/Kg body weight IVIG for 5 consecutive days, given in monthly cycles. The only downside of IVIG therapy is its high price. Therefore, clinicians should balance efficiency versus cost in deciding whether or not to treat certain conditions with IVIG.