Why are lung allografts more vigorously rejected than hearts?

The fact that lungs are more prone to rejection than hearts was studied in a fully allogeneic rat strains combination. Between these strains, lung rejection was significantly faster than that of the heart (mean survival time of 4.0 versus 6.8 days). The mucosal immune system transplanted within the lung allograft was found to intensify the recipient's immune response in several ways. First, it facilitates the infiltration of the graft by recipient lymphocytes. Secondly, it provides a strong stimulus for the local immune response in the graft itself, and lastly, the dissimination of donor lymphocytes from the graft into the recipient generates a systemic immune response.