Diabetes type 1 is usually due to defect in production and release of Insulin by Pancreas(Specifically Beta Cells). (Family History is usally present in Type 2 DM. )
There are cells in our body which are responsible for our bodie's immunity. This immunity is provided by antibodies which help to neutralize or destroy the foreign Contaminant or Bacteria etc. Sometimes the cells which produce these antibodies, produce them against a person's own body, like in Case of Type 1 DM where these Antibodies are produced against Beta Cells of Pancreas which produce Insulin , because of distruction of Beta Cells there is less or no insulin production by Pancreas therefore our body cannot utilize the sugars which we take in our diet (Insulin is responsible for Uptake of Sugar in our body by resting muscles and Fat cells etc. ) Therefore the Person who has type one DM, has to take Insulin.
Type one can also occuring type 2 DM where due to increased stress on Beta Cells of Pancreas eventually there is Failure of Beta cells and then Type 2 DM is converted into Type 1 DM..