I've been researching P-glycoprotein, a membrane glycoprotein that is associated with resistance to a variety of drugs, including many chemotherapy drugs used in cancer. There is evidence that p-glycoprotein levels can can be expressed up to seven times more numerously in tissues of individuals who are blood type A which may go along way towards explaining why it appears that type A's with cancer who receive chemotherapy often do not have as beneficial an effect as the other blood types.
Obviously finding ways to modulate P-glycoprotein would be very desirable, especially if they could be administered during chemotherapy. So far there appears to be a variety of flavones and alkaloids found in nature that up-regulate or down-regulate P-glycoprotein, so perhaps a nutritional application may be possible. Much more work is needed, but from the biochemical aspect, it is quite fascinating. From a broader perspective, the potential that P-glycoprotein inhibitors may have for the treatment of various immune disorders should also be investigated.