Red blood cells express multiple molecules, including hundreds of different proteins and carbohydrates on their surfaces. These surface molecules vary among individuals. Some of these surface molecules are not present on some people's red blood cells. Other surface molecules exist in slightly different forms in different people. These variations are important during transfusion of red blood cells because cell surface molecules on the transfused red blood cells that are not present on a patient's red blood cells are “foreign” and can induce an immune response in the patient.