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.
From there, the blood flows through the right and left pulmonary arteries into the lungs. In the lungs, oxygen is put into the blood and carbon dioxide is taken out of the blood during the process of breathing. After the blood gets oxygen in the lungs, it is called oxygen-rich blood.