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 used to restore oxygen-carrying capacity in people with anemia due to trauma or other medical problems, and are by far the most common blood component used in transfusion medicine. Historically they were transfused as part of whole blood, but are now typically used separately as RBCs and plasma components. The process of identifying a compatible blood product for transfusion is complicated. Side effects can include allergic reactions including anaphylaxis, red blood cell breakdown, fluid overload, infection, and lung injury. Giving incompatible RBCs to a person can be fatal. With current testing methods in high-income countries the risk of infection is