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A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system. Some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens that stem from one allele (or very closely linked genes), collectively form a blood group system.^[1] Blood types are inherited and represent contributions from both parents. A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT).^[2]

Many pregnant women carry a fetus with a different blood type from their own, and the mother can form antibodies against fetal RBCs. Sometimes these maternal antibodies are IgG, a small immunoglobulin, which can cross the placenta and cause hemolysis of fetal RBCs, which in turn can lead to hemolytic disease of the newborn, an illness of low fetal blood counts which ranges from mild to severe.^[3]

ABO blood group system

ABO blood group system - diagram showing the carbohydrate chains that determine the ABO blood group

The ABO system is the most important blood-group system in human-blood transfusion. The associated anti-A antibodies and anti-B antibodies are usually "Immunoglobulin M", abbreviated IgM, antibodies. ABO IgM antibodies are produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses. The "O" in ABO is often called "0" (zero/null) in other languages.^[16]

Red blood cell compatibility table

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

^{SOME SAMPLE OF BLOOD TYPES PICTURES(Pic By AHNAF Kasaraaini)}