While most of the components in blood are generally the same in everyone, its minor differences can make someone who receives blood that is incompatible with their blood type suffer a severe reaction.
A protein or antigen that is found on the surface of red blood cells is what makes blood types different. It acts as a chemical marker whose function is to detect the cells that belong to the body and the cells that do not.
These proteins are classified into two types: A and B. These two letters represent the two main blood types. People who do not have A or B antigens fall into the O blood type, and those who have both proteins belong to the AB group.
Each group is subdivided into either Rh positive or negative blood types, which determines how compatible the groups are with one another. The Red Cross in Spain explains that Rh negative blood groups can donate to people from their same blood group, whether they are positive or negative; while Rh positive blood types are only compatible with other Rh positive blood types.
85% of people are Rh positive, while 15% are Rh negative.
Universal donors are people who have type O Rh negative blood. Their blood type is a compatible with all blood groups, allowing them to donate their blood to anyone without causing an aggressive reaction in their bloodstream. These people can only receive blood from someone from their same blood group, however.
The following table shows the compatibility between donors and recipients.
Read also: Donating blood is an act of generosity