Type 1 diabetes mellitus is an autoimmune disorder characterized by the destruction of insulin-producing β-cells of the pancreatic islets of Langerhans’ and lack of endogenous insulin. Susceptibility to type 1 diabetes mellitus is influenced by both genetic and environmental factors. It is generally believed that the environmental agents, such as viral infections, dietary factors in early infancy or climatic influences, trigger disease development in genetically susceptible individuals. The HLA genes are the strongest determinants for type 1 diabetes – there are some susceptibility genes, especially HLA-DR3, -DR4, HLA-DQ2, -DQ8. β-cells destruction is caused by T cell mediated autoimmune reaction. Effector mechanisms probably are dominated by cell-mediated β-cell destruction. The phase prior to the onset of insulin deficiency is heralded by the development of circulating autoantibodies against multiple antigens (insulin, β-cell antigens).