Blood group antigens are carbohydrates that are attached to proteins or lipids. An antigen is a substance foreign to the body that causes an immune response. An immune response occurs when antibodies, which are proteins in your immune system, are summoned to attack an antigen.
Blood that reaches the lungs travels throughout a network of small blood vessels, where oxygen moves into the blood and carbon dioxide moves out of the blood, according to the National Heart, Lung and Blood Institute.
Before blood can begin its journey around the body again, it must travel to the lungs to release carbon dioxide and take in oxygen. The right atrium accepts the oxygen-poor blood and delivers it to the right ventricle, which pushes the blood through an artery to the lungs. The blood then returns in a vein from the lungs to the left atrium, where it waits before passing to the left ventricle and back through the aorta again.