We compare two kinds of twins. There are twins that are genetically identical, and there are twins that are non-genetically identical. If something is caused by an environmental factor, there should be no difference between identical twins and non-identical twins. If there's a difference, it suggests that genetics plays a role. These studies have been done for a number of autoimmune diseases, and the answer has always come up about the same.
Genetic components represent something in the order of half of the risks.
So genetics plays an important role. One group of genetic factors is particularly important. In general you cannot accept a kidney or heart from someone else unless we dampen your immune response. There clearly are significant physical differences between different people. And we call the substance that causes that difference histocompatibility complex.