1. **Stating the problem:** Which blood type can be transfused into recipients of any genetic makeup with negligible ill effects? 2. **Background:** Blood types are classified mainly by the ABO system: A, B, AB, and O. - Type A has A antigens on red cells and anti-B antibodies in plasma. - Type B has B antigens and anti-A antibodies. - Type AB has both A and B antigens but no anti-A or anti-B antibodies. - Type O has no A or B antigens but has both anti-A and anti-B antibodies. 3. **Important rule:** - The universal donor blood type is the one that can be given to any recipient without causing an immune reaction. - This is because the donor's red blood cells lack antigens that the recipient's antibodies might attack. 4. **Applying the rule:** - Type O blood has no A or B antigens, so it will not be attacked by anti-A or anti-B antibodies in the recipient's plasma. - Therefore, type O blood can be transfused to any recipient safely. 5. **Conclusion:** The blood type that can be transfused into recipients regardless of their genetic makeup with negligible ill effects is **type O**. **Final answer:** 5. O