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.
Oxygen-poor blood is brought back to the lungs by the pulmonary artery. The pulmonary artery divides into branches that parallel the bronchial tubes and ends in a network of pulmonary capillaries (tiny blood vessels) within the walls of the small air passages and alveoli of the lungs. The pulmonary veins carry oxygen-rich blood away from the lungs.