Question

What does it mean to have an O positive blood group?

Answer 1

Hint: A person with blood group O is said to be a universal donor whereas a person with AB blood group is called a universal recipient. A person who has blood group O does not have any antigens present on the plasma membrane of his red blood cells (RBC).

Complete answer:
There are three alleles coding for the blood group in humans and thus it comes under multiple allelism.
The allele ‘$I^A$’ causes A antigen, allele ‘$I^B$’ causes B antigen whereas allele ‘i’ results in no antigen.
The alleles ‘$I^A$’ and ‘$I^B$’ are completely dominant over ‘i’ but codominant over each other.
Another type of blood grouping is seen in humans known as the Rh blood grouping.

When allele ii are present that person has O blood group. And a person having O blood group is said to be O positive only when they have Rh antigen present in his blood. O blood group can be given to persons with blood groups A, B, AB, or O which means O blood group can be given to all the other blood groups. This is because O group blood does not have any antigens on the membrane of RBC and thus are not attacked by the antigens released by people having blood groups A, B, AB, or O.

But a person with an O blood group can only receive blood from another O group person. This is because the O group produces antibodies A and B and will destroy RBC that are given by people with blood groups A, B, and AB.
                               

Note:
O blood group is the universal donor because the blood has no antigens present in it. Because there are no antigens present on the plasma membrane of the RBC in blood, antibodies A and B both are present in the plasma of such people.