Chronic rejection of is one of the main causes of death after heart transplantation. The mechanisms whereby immune cells from the host reject the transplanted hearts are not fully understood. A certain type of white blood cells, called B lymphocytes, is often seen directly in rejected hearts, yet their function remains unknown. In our studies, we will isolate these particular cells directly from samples collected from patients who have experienced rejection and study them. We hypothesize these white blood cells fuel the immune response to heart transplants and largely contribute to their rejection.