Red blood cell endothelial cells. Exposure of red blood cells to physiological levels of shear stress activates nitric oxide synthase and export of nitric oxide, which may contribute to the regulation of vascular tonus. Red blood cells can also produce hydrogen sulfide, a signalling gas that acts to relax vessel walls. It is believed that the cardioprotective effects of garlic are due to red blood cells converting its sulfur compounds into hydrogen sulfide. Red blood cells also play a part in the body's immune response: when lysed by pathogens such as bacteria, their hemoglobin releases free radicals, which break down the pathogen's cell
Packed red blood cells though it has other names and is commonly just called "negative" or "positive." Unlike the ABO antigens, a recipient will not usually react to the first incompatible transfusion because the adaptive immune system does not immediately recognize it. After an incompatible transfusion the recipient may develop an antibody to the antigen and will react to any further incompatible transfusions. This antibody is important because it is the most frequent cause of hemolytic disease of the newborn. Incompatible red blood cells are sometimes given to recipients who will never become pregnant, such as males or postmenopausal women, as long as they