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 a health system. In the United Kingdom they cost about 120 pounds per unit. A number of other versions also exist including whole blood, leukocyte reduced red blood cells, and washed red blood cells. More than 100 million units of blood are collected each year around the world, and about 50% of these are given to people in high income countries. In low-income countries, the majority of blood transfusions (up to 65%) are given to children under 5 years of age to treat severe childhood anemia. Another major use of blood in low income countries is to treat pregnancy-related complications.