Why does our body not reject the insulin made from genetically modified organisms like bacteria?

Mimics our insulin: The human Insulin genome is genetically transfered into the bacteria. Then it produces the insulin, identical to human Insulin so there is no reaction against it (humulin). The Insulin from that source is purified to eliminate bacterial proteins and so it is non-reactive.

Similar structure: Hi the engineered insulins are VERY similar in structure to native human insulin. The human insulin gene is rearranged so that the engineered protein differs from regular human insulin by only 1 amino acid. The change is significant for how the insulin molecules behave at acid/neutral pH or how they fail to aggregate, but not enough to be recognized as different by the immune system. Fortuitous!