Answer: Difference between animal and human insulin and/or not being properly purified.
Step-by-step explanation:
Insulin is a hormone that regulates the metabolism of secreted by the β-islet of Langerhans cells of the pancreas. It that allows blood sugar, known as glucose, to pass into cells. Glucose comes from the food and drinks and it is the body's main source of energy. So, insulin plays a key role in maintaining proper blood glucose levels.
Animal insulin is produced by the pancreas of animals, usually cows and pigs. Then, the insulin is purified to reduce the chance of the insulin user developing a reaction to the insulin.
Each organism has its own genetical identity and any foreign particle is treated as an antigen. And, there is small chemical difference between insulin produced in animals and insulin produced in humans. That difference, caused by a different sequence of peptides, can be recognized by the immune system as something foreign and as such can be a threat to the body. And that would trigger an immune response. Also, the response can be because it hasn't been properly purified and contains other compounds or proteins that are also foreign.