Roughly 80% of cases of type 1 diabetes are autoimmune (where type 1 diabetes refers to diabetes caused by an absolute lack of insulin).
Additionally, at least 15% of cases of type 2 diabetes have an autoimmune component.
What does this mean?
Antibodies are produced by cells made in the bone marrow (the immune system is mostly made of cells made in the bone marrow, thymus, and spleen). In order for a person to develop an autoimmune disease, xe needs to have a semi-functional immune system, which means xe needs to have bone marrow.
The only medically likely transmission of type 1 diabetes has occurred through a bone marrow transplant in which the donor was a type 1 diabetic. In the most published case, the recipient was the sister of the donor. She received the transplant at the age of 25, when antibody negative, and was diagnosed with diabetes at age 29, at which time she was antibody positive. http://www.ncbi.nlm.nih.gov/pubmed/8098394
A longer summary of all cases of bone marrow donation by type 1 diabetes is here; the short story is that most recipients do not develop type 1 diabetes but a lot more than could be randomly expected to, do.
The role of the bone marrow in creating autoimmunity means that bone marrow transplants have the possibility of causing auotimmunity- but they also mean that bone marrow transplants have the possibility of curing autoimmunity. What happens to a person's autoimmune disease when you kill off the person's bone marrow, and then give xem a bone marrow transplant?
Answer: Just about anything. In a fairly famous series of clinical trials, doctors removed and radiated bone marrow,killed off the bone marrow left in the body, and then replaced the bone marrow in the patients, who were newly diagnosed type 1 diabetics. Some of them no longer had diabetes. In some, the autoimmunity and diabetes reoccurred. In some, it didn't go away in the first place.
Currently, if you are a newly diagnosed adult type 1 diabetic (antibody positive and never been in DKA), you are eligible to participate in such a trial at the hospital I go to.
Autoimmunity also means that immunosuppressants have some chance of preventing, treating, or even reversing the disease. Type 1 diabetics who go on immunosuppressants rarely have a full remission but are more likely to make more of their own insulin. In some type 1 diabetics, the autoimmune process seems to have been a one time deal- the antibodies came, they marked the territory, the macrophages came and gobbled up the beta cells, end of story. In other cases, it seems to be more of an ongoing battle- the antibodies came, they marked the territory, the body went and made more beta cells, the macropaghes came and ate the marked beta cells, repeat ad infinitum. In the latter case, immunosuppression may be more likely to make a difference.
Another reason to care if it's autoimmune: the more autoimmune diseases you have, the more you're likely to develop.