IVIG is a blood/plasma product derived from blood donors IgG. It is an accumulation of antibodies from multiple donors. Each batch can contain IgG from the serum of between 1000 and 1500 donors.
It is collected and given via IV, much like a blood transfusion. It is administered to patients that may be lacking needed protections to particular diseases. Or those with a weakened immune system.
It is administered in multiple IV sessions, and dosage based on body weight as well as other factors.
In some autoimmune patients it is given to help raise the amount of Red Blood Cells. In some it is used to stop the white cells.