Effective against extracellular antigens including bacteria, viruses, protozoans, fungi, parasites, and toxins when they are outside cells.Antibodies or Immunoglobulins (Ig)Classes: IgG, IgM, IgA, IgE, IgDStructureVariable region: Part that combines with antigenic determinant of antigenConstant region: Responsible for activities of antibodies like activating complement or attaching to various kinds of WBCs.