Not all humans are same and they have different blood groups.
There are of 4 types of blood groups broadly as A, B, AB & O group in humans.
This means a person with one blood group cannot receive blood from any one randomly. But instead, he needs a select group from select persons.
If blood is administered randomly, the recipient would be susceptible for immune reaction which can sometimes be fatal.
Hence, blood from particular group is required for transfusion. So, blood is collected in advance and stored in blood banks for emergency use.
The blood group of a person can be identified by immune reactions to determine the group.
These groups also have a bifurcation as positive blood group (+) and Negative blood group (-) as A+ve, B+ve, AB+ve, O+ve and even similarly as A-ve, B-ve, AB-ve, O-ve groups.
So the underlying difference of positive (+) and negative (-) arises due to a substance termed Rhesus antigen Rh or rhesus factor especially type D-antigen.
If the blood of any person has this rhesus factor, he is assigned his blood group with a positive sign beside like B+, A+, O+, etc..
If this factor is not present in his blood, he is assigned a blood group with a negative sign as B-, A-, O- etc.
When a blood transfusion is done with incompatible blood group, there is a severe reaction inside the body leading to death. This instantaneous response occurs due to conflicting blood groups like A, B, AB & O are transfused, but on the other hand, the incompatibility with positive and negative associations doesn’t show immediate reactions but show reactions after any subsequent incompatible infusions.
Rhesus Rh factor is again of different types like C, D, E, c, d, and e. Any person with C antigen doesn’t have c-antigen and that with D-antigen doesn’t have d antigen and similar in E also. So a person can have a blood group with any of the above rhesus factors but Rh-D is widely prevalent and hence persons with Rh-D are termed as +ve and those without Rh-D are termed as -ve.
Rh factor and pregnancy: When a mother has Rh-ve blood group and if the baby in the womb has Rh+ve group then there are chances of development of incompatible antigens for that blood group in the mother but may not affect the present baby. But in the second pregnancy, if the baby has Rh+ve blood group, then there are compatibility issues as the antigens already formed due to first pregnancy is present. So this may lead to abortion or death of the baby of the second or any further gestation.