Question

why is type o rh - negative the \universal donor?\
the o and rh surface markers on the donor red blood cells block immune reactions in the recipient.
no surface markers are present on the o rh - negative blood cells to react with the recipients immune system.
three surface markers are present on the donor blood cells (a, b, and o), which block immune reactions in the recipient.
three surface markers are present on the donor blood cells (a, b, and o) that are recognized as compatible in all recipients.
more people have type o blood than any other blood type.

Explanation:

Blood type is determined by surface markers (antigens) on red blood cells. Type O Rh - negative blood cells have no A, B (for blood type) or Rh (Rhesus factor) surface markers. When a recipient receives blood, their immune system reacts to foreign surface markers. Since type O Rh - negative has no such markers, it won't trigger an immune reaction in most recipients (assuming proper cross - matching for other minor antigens), making it a "universal donor" in emergency situations (though in practice, more comprehensive matching is preferred).

Answer:

No surface markers are present on the O Rh - negative blood cells to react with the recipient's immune system.