Type O refers to a blood type characterized by the absence of A and B antigens on the surface of red blood cells. It is one of the four main blood types in the ABO blood group system, which also includes Type A, Type B, and Type AB. Type O blood can be further classified into O positive and O negative, depending on the presence or absence of the Rh factor. Individuals with Type O blood are often considered universal donors, meaning their blood can be transfused to patients of any blood type. This is particularly important in emergency situations where there may not be time to determine a patient's blood type. However, people with Type O blood can only receive blood from other Type O donors.