When the body is invaded by antigens, the immune system forms substances called?

Antibodies. A. k. a. immunoglobulins. They are produced by a group of white blood cells called B lymphocytes (or B cells). There are 5 classes of immunoglobulins; IgM, IgA, IgE, IgG and IgD. The classes of antibody diifferent both in their constant region and in their activity. IgG, the most common antibody, and is found mostly in the blood and tissue fluids, and IgA is localized in the mucous membranes which line the respiratory and gastrointestinal tracts.