The Immune system is a complex network of cells (such as lymphocytes) and organs that work together to defend the body against foreign substances (antigens) such as bacteria, a virus or tumor cell.When the body discovers such a substance several kinds of cells go into action in what is called an immune response.ymphocytes are divided mainly into B and T cells. 1  B lymphocytes produce antibodies-proteins (gamma globulins) that recognize foreign substances (antigen) and attach themselves to them. 2  B lymphocytes (or B cells) are each programmed to make one specific antibody.