How blood type works

Our blood has certain markers on the surface of red blood cells. They are called Antigens (kind of chemical compound). Blood is categorized based on the presence of the antigen.  

If red blood cells have	then Blood group is
A antigen	A
B antigen	B
Neither A nor B antigens	O
A and B antigens	AB

Now you may wonder what does “+” and “-“ sign in the blood group indicate. There is one more antigen called “Rh”. The presence of Rh antigen indicates blood group is positive and the absence of Rh antigen indicates blood group is negative. We can revise the above table as shown below.

If red blood cells have	and Rh antigen present	then Blood group is
A antigen	Yes	A+
A antigen	No	A-
B antigen	Yes	B+
B antigen	No	B-
A and B antigens	Yes	AB+
A and B antigens	No	AB-
Neither A nor B antigens	Yes	O+
Neither A nor B antigens	No	O-

A blood type will have antibodies to fight against the antigens which are not present in that blood. For example, “A-” blood type will have antibodies to fight against “B antigen” and “Rh antigen”.  So, “A-“ type will accept only “A-“ and “O-“ blood types. The complete blood type compatibility is given below.

Blood types that match

A- blood  <== A-, O-   

A+ blood  <== A-, A+, O-, O+  

B- blood  <== B-, O-  

B+ blood  <== B-, B+, O-, O+  

AB- blood <== AB-, O-  

AB+ blood <== AB-, AB+, A-, A+, B-, B+, O-, O+  

O- blood  <== O-  

O+ blood  <== O-, O+