Diabetes mellitus is not a single disease.
Type I diabetes involves the damaging of the Islets of Langerhans in the pancreas, such that insulin is either not produced at all or produced in limited quantities.
Current thinking posits an autoimmune response that damages the pancreas. There is evidence of a genetic component. Usual patients are prepubertal, but adults can develop the condition. Characterized by high blood glucose levels, and higher than normal A1C hemoglobin results.
Type II diabetes involves insulin resistance. The pancreas makes insulin, the receptor sites for insulin "get the message", cells do not respond so serum glucose levels remain high until the kidneys are able to reduce the level. Primary causes are environmental, largely a complex of dietary choices, total calories, low exercise levels, and obesity. It is believed there is a genetic component as well. Age of onset can be as low as 10 years of age. Very common in developed countries in older adults.
Either type poorly controlled results in tissue damage: retina (diabetic retinopathy), neuropathy, kidneys, vascular system, skin. Basically it means that the diabetic has a lower life expectancy compared to non-diabetics. We do not need more details here really.
So,answer: not immunocompromised unless you have a different, possibly non-standard definition.
http://www.webmd.com/diabetes/type-1-diabetes-guide/default.htm
http://www.webmd.com/diabetes/type-2-diabetes-guide/type-2-diabetes