The immune system mainly consists of reticuloendothelial system, i. e. bone marrow, liver, lymph nodes, spleen and thymus. B-cells that are stimulated by an antigen, undergo rapid multiplication and produce cells. These cells are capable of producing antibody. They are called plasma cells, the B-cells which are sensitized by a specific antigen retain the memory and whenever the same antigen enters into the body, the B-cells immediately proliferate and become plasma cells to produce high concentration of antibodies. Immune response is mainly divided into two types humoral or antibody mediated immunity and cell-mediated immunity. In certain diseases like tuberculosis, leprosy or against certain antigens one cannot demonstrate specific antibody. The reason is, such diseases or such antigens do not stimulate the B-lymphocytes but they stimulate directly T-lymphocytes. Those T-lymphocytes, which are stimulated by such antigens or diseases, they become activated and produce a variety of soluble, protein like factors of low molecular weight, which are called lymphokines or cytokines.