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.
How it works: The process of oxygen getting into the blood goes like this. First we inhale air with oxygen in it. Little pockets in the lung called alveoli fill with the air and the oxygen diffuses into capillaries. This oxygenated blood then goes back to the heart where it is pumped around to the rest of the body.