This class of antibodies is really busy and is produced in large amounts in secondary immune responses. IgG coats microbes, including viruses, and targets them for phagocytosis by macrophages and neutrophils. IgG also activates natural killer cells. IgG is the only class of antibodies that can cross the placenta and is important in providing passive immunity to the newborn. This class of antibodies is like the sea star of the immune system .
Sea stars have five arms.
IgM is the first class of antibodies that is produced when the immune system first meets a microbe. Many diagnostic tests that determine whether or not you are infected with something are actually measuring a rise in this kind of antibody. This rise, if significant, is called seroconversion . CMI is also important in transplantation reactions. T cells are also formed in the bone marrow but they track to the thymus gland located under the breast bone.