Blood Types

There are four types of blood in the human body. They are O, A, B, and AB. As with the chart above each type has an antigen on the surface of every RBC except type O. This is the reason type O is the universal donor however type O people can only receive type O blood. The RBC's won't present an antigen for the immune system to recognize as a foreign substance in the body. Conversely type AB is the universal recipient because it has all the antibodies present, types A, B, AB, and O can all be given to someone with AB blood. The last part of this is the RH factor. This is where + or - come into play. Each blood type has a RH component so there is O+/-, A+/-, B+/-, and AB+/-. The plus simply means there is a marker on the RBC that is not present in the negative. This makes O negative as the universal donor and AB+ as the universal recipient. This chart summes everything up.

Recipient[1]	Donor[1]
	O−	O+	A−	A+	B−	B+	AB−	AB+
O−	Y
O+	Y	Y
A−	Y		Y
A+	Y	Y	Y	Y
B−	Y				Y
B+	Y	Y			Y	Y
AB−	Y		Y		Y		Y
AB+	Y	Y	Y	Y	Y	Y	Y