Why do some people's bodies reject organ transplants even with immunosuppressants?

I've known someone who went through a transplant rejection despite being on immunosuppressants, and it's frustrating because the body's immune system is remarkably stubborn. Even with medication dampening the immune response, your body can still recognize the new organ as "foreign" if there's a mismatch in tissue types - basically, your cells have molecular markers (called HLA types) that need to be compatible enough with the donor's, and sometimes they just aren't close enough. Some people also develop antibodies against the transplant over time, which the drugs can't always prevent. It's why matching donors and recipients carefully beforehand, and sometimes using stronger or different immunosuppressants, can help reduce rejection rates.