I know that they used to use pork insulin. Why didn’t the human body reject it?

With some very occasional exceptions, pork insulin was well tolerated by the human body, primarily because pork insulin is almost identical to human insulin. It was certainly a significant improvement over beef insulins.

In fact, over the years, some of the rare problems were shown to be due not to the insulin itself but to residual impurities from the manufacturing process. As the DNA story developed, it became possible to synthesise human insulin DNA and to introduce this into bacteria or yeast cultures as a way of producing insulin in large quantities. These semisynthetic insulins were exactly like the human molecule and were, of course, much purer and less costly to manufacture.

DOB