Blood group systems
Blood group (or blood type) is a term used to clasify blood based on the presence or absence of a specific antigen on red blood cells surface. According to the International Society of Blood Transfusion, there are about 30 human blood group systems recognized so far .
ABO blood group system
The most common blood group system being used nowadays is ABO blood group system. In this system, there are 4 classification of blood types. They are group A, B, AB and O. These groups are classified according to the presence or absence of ABO antigen.
|Blood group||Antigen(s) present on erythrocytes||Antibodies present in plasma||Genotype(s)|
|A||A antigen||Anti-B||AA or AO|
|B||B antigen||Anti-A||BB or BO|
|AB||A and B antigens||none||AB|
|O||none||Anti-A and B||OO|
Rh blood group system
Rh blood group system is the second most important system in determining blood groups. It is based on the presence or absence of D antigen on the red blood cells surface. If a blood transfusion containing Rh-positive blood is given to a Rh-negative individual, who does have the Rh antigen, their immune system will attack the blood cells as it will be seen as a danger and foreign. This leads to blood cells being destroyed. 
- ↑ Table of blood group systems, ISBT (August 2008). Retrieved on 15 November 2010.
 https://www.britannica.com/science/Rh-blood-group-system [cited 18/10/16]