How do you determine someone's blood type in lab?
Each well has either A antibodies, B antibodies or RH antibodies
If a well agglutinates, that antigen is present (i.e. if the well with A antibodies agglutinates, A antigens are present and they are sticking to the antibodies- which are not normally present in type A blood- therefor it is type A blood)
If it clumps, that's what type it is