Two persons with the blood type O+ (positive) can marry. Any blood type (A, B, AB, O) may marry and have healthy offspring with any other blood type. The couple will need to identify their blood types and take precautions not to transmit disease through sexual contact or via injury with its sharp objects. Blood is made up of many different substances that determine a person's unique traits. Blood groups are found on red blood cells and some other body fluids and tissues. A person's blood type is determined by the presence of antibodies against specific antigens (pieces of protein) found on the surface of blood cells. There are three major blood groups: A, B, and O. Each person has cells from all three groups; therefore they can be typed as "ABO positive." The ABO blood group system was first described in 1921 by Drs. Karl Landsteiner and Henry Dale. They showed that people who lack anti-B antibodies have worse symptoms after receiving blood transfusions than those who already have these antibodies. Also, people who receive blood products must be typed before they are given the product to ensure that it contains the right type of blood cells. About one in four people are type O, called "zero" blood types because they have no antibody resistance to infection.