A blood product normally used to treat immune disorders and a type of leukemia may also slow or stop mental decline in people with Alzheimer’s disease, researchers reported yesterday at an Alzheimer’s conference in Madrid. The product is called IVIg (pronounced EYE-vig), for intravenous immunoglobulin, also known as gamma globulin. Made from pooled blood plasma, it is a thick soup of antibodies, the proteins made by the immune system to get rid of unwanted substances. It has been used for 30 years for other diseases and is dripped into a vein like a transfusion. But the findings in Alzheimer’s are...