The basis of testing in blood transfusion is to establish a patients blood group (A,B,O,AB), their RhD type (positive or negative) and whether they have any atypical antibodies in their plasma which could react with any transfused red blood cells (an antibody screen – positive (and then you would determine the antibody) or negative).
Most of the work is now automated as process around 100 samples a day and that wouldn’t be possible by hand! The analyser also removes an possible human error.
The reagents for blood grouping are antibodies to the A, B and O blood group antigens which are carbohydrates that stick out on the red cell surface. If the patient is group A they will react with antibodies to A red blood cells causing them to clump together and give a positive reaction which allows us to determine the blood group is A. The same method is used to determine the RhD type.
To detect any atypical antibodies (these are only produced after exposure to foreign red blood cells either through transfusion or pregnancy) the patients plasma is tested against a set a red cells with known antigens sticking out on their surface. If this is positive we use more cells to determine the type of antibody present. It is a process of elimination!
Some other red blood cell antigens are called, K, Jka, Fya, P1, C, E, c, e and there are hundreds more!! Around 25-30 are clinically significant though.
Thanks for your question!