Beyond the ABO system, there is another blood characteristic that carries significant medical weight: the Rhesus factor, usually shown as Rh+ or Rh- on your blood group result. When you see someone described as A positive or O negative, the 'positive' or 'negative' refers to the Rhesus factor. The Rh factor is a protein that may or may not be present on the surface of your red blood cells. If you have this protein, you are Rh positive (Rh+). If you do not, you are Rh negative (Rh-). About 85% of people are Rh positive. Being Rh negative does not make you sick or abnormal in any way — it simply means your blood cells do not carry this particular protein. The Rh factor becomes critically important in one specific situation: pregnancy. If a mother is Rh negative and her baby is Rh positive (having inherited the Rh gene from the father), the mother's immune system may produce antibodies against the Rh protein if any of the baby's blood enters her bloodstream — typically during delivery, a miscarriage, or certain procedures. During a first pregnancy, this usually causes no problem. But in a second or subsequent pregnancy with another Rh positive baby, the mother's immune system is now primed. The antibodies she developed cross the placenta and attack the baby's red blood cells. This condition, called Rh incompatibility or haemolytic disease of the newborn, can cause severe anaemia, jaundice, brain damage, or stillbirth in the baby. This risk is entirely preventable. An injection called anti-D immunoglobulin, given to an Rh negative mother after delivery or exposure, stops her immune system from developing those antibodies. But this only happens if the doctor knows her Rh status — which means she needs to know her blood group, including the Rh factor, before and during pregnancy. For young Nigerians planning their future, this is one more reason why knowing your complete blood profile matters now, not later.