A highly discriminatory immune system is fundamental to survival. The Immune system has a powerful collection of defense mechanisms to protect against potential invaders that would otherwise take advantage of the rich source of nutrients provided by the vertebrate host. Unlike the cells of liver, heart or lungs, the cells of the immune system are scattered throughout the body. They are present in the spleen, lymph nodes, bone marrow, and thymus & circulate through the blood and lymphatic fluid. The first line of defense is provided by the intact skin and mucous membranes of the body. There are a group of proteins (immunoglobulins) present in normal adult, they are of  five types IgG, IgA, IgM, IgD, IgE. Amongst these IgG and IgA play important role in Periodontitis as IgG are the main circulating antibody in the blood and can pass from blood into tissue spaces whereas IgA protection to mucosal surfaces.