When insulin is injected, the body views it as a foreign substance and the immune system starts to work.  This is true for humans using r-DNA/GE/GM insulin which is an exact replica of the insulin one's body produces, and also for dogs on porcine insulin, which is an exact match to native canine insulin. What is true for them is true for other animals, who can also have antibody resistance to insulins not like their own. 
Even if the injected insulin is "just like", the immune system is smart enough to know it was not produced by your body's pancreas, and thus it proceeds to form antibodies.  It's the antibody process which helps fight off colds and other illnesses and infections. Most of the time this is a helpful function of the immune system; the antibodies created either assist in the body's recovery from its problem or those which were created really cause no serious problems. 
Most humans with diabetes have antibodies to human insulin in their systems; most of the time it creates no serious problems with their effective use of the drug. Those who use analog insulins also form antibodies to these insulins-at about the same percentage for those using human insulins. As with most people, most cats and dogs with diabetes have no serious antibody related problems. 
Sometimes, especially in the case of those using insulin, it is a serious problem, and the body's immune system sets out to destroy the "invader" insulin, just as it does to cold and flu viruses.  When this happens to injected insulin to the degree that most of it never reaches your bloodstream to do anything about controlling blood glucose, it is officially known as insulin resistance.  When this happens, dosage increases don't matter--the immune system just goes into higher gear and effectively destroys the additional injected insulin.
It is also possible for the insulin itself to be accepted by the body, but there are other proteins such as protamine present in the suspension of the insulin which cause the antibody problems. 
A look at insulin resistance. This animal receives two injections of Lente insulin daily. (Animal and insulin species not stated--blood glucose meansurements in mmol/l (non-US) values.) They are not successful in reducing the blood glucose levels.
- ↑ Bernstein, Richard (2000). The Laws of Small Numbers-Insulin Dose Absorption. diabetes-normalsugars.com.
- ↑ Mooney, Carmel T. (2003). Unstable Diabetics-Solving the Problems. WSAVA.
- ↑ Regulating & Monitoring a Diabetic Cat Using Insulin. Drs. Foster & Smith-Pet Education.
- ↑ 4.0 4.1 Brooks, Wendy C.. The Hard to Regulate Diabetic Dog. Veterinary Partner.
- ↑ The Immune System. Drs. Foster & Smith Pet Education Library.
- ↑ 6.0 6.1 6.2 Nelson, Richard (2006). Selecting an Insulin for Treatment of Diabetes Mellitus in Dogs & Cats-Page 40. OSU Endocrinology Symposium.
- ↑ Richards, Mike. Insulin Use in Dogs-Insulin Antibodies. Richards, Mike-Vet Info 4 Dogs.
- ↑ Vetsulin-FAQs. Intervet.
- ↑ Vetsulin--Antibodies. Intervet.
- Allergy MooreVet
Though this page deals with allergies, it presents a good explanation of how antibodies are created and the process of how the immune system works.
- Porcine vs Bovine Insulins NAVC