IgG stands for immunoglobulin G, a type of antibody. Antibodies are proteins made by the immune system to fight viruses, bacteria, and other foreign substances. A CSF IgG index measures the levels of IgG in your cerebrospinal fluid. High levels of IgG can mean you have an autoimmune disorder. An autoimmune disorder causes your immune system to attack healthy cells, tissues, and/or organs by mistake. These disorders can cause serious health problems.