A once obscure medical treatment is seeing new popularity thanks to an improved understanding of the role the immune system plays in conditions as varied as obsessive-compulsive disorder (OCD) and Alzheimer's disease. Some worry that supplies of intravenous immunoglobulin (IVIG), which is made of blood plasma from donors, may run short if a clinical trial confirms its effectiveness at slowing the progress of Alzheimer's.

IVIG contains an antibody known as IgG that helps to ward off infection, modulates the immune system and reduces inflammation, although the full extent of how IVIG works remains unknown. When IVIG was first approved commercially in the early 1980s—it was prescribed to replace antibodies in patients with primary immunodeficiency disease (PIDD) and, later, was used to regulate the immune system in autoimmune conditions such as multiple sclerosis. Today IVIG has over 100 off-label indications, which represent the fastest-growing sector of its market.

Among these are an emerging crop of psychiatric illnesses, including some forms of schizophrenia and OCD, that may have autoimmune causes. IVIG is now often prescribed to patients with autoimmune encephalitis, a group of rare conditions in which antibodies attack the brain, causing such symptoms as psychosis and catatonia. And there are clinical trials, due to be completed in 2016, studying the effects of IVIG on children with sudden-onset OCD, which some researchers believe can be caused by antibodies to Streptococcus bacteria crossing the blood-brain barrier.

Researchers are also hoping that IVIG may be able to slow the progression of Alzheimer's. A recent study at Weill Cornell Medical College suggested that IVIG might reduce the buildup of aberrant proteins in the brain and quell the damage caused by inflammation. Although IVIG's Alzheimer's application is still in late-stage trials, the market is bracing itself for a surge in demand—up from 7 to 12 percent a year—if approval is met. “It's a real concern because [IVIG] is not just a pill that you make. It's not an unlimited resource,” says Jordan Orange, professor of pediatrics, pathology and immunology at the Baylor College of Medicine. Doctors are urging one another to reexamine how they prescribe IVIG treatments and to seek alternatives in cases where the benefits are less clear.

COMMENT AT ScientificAmerican.com/jan2013