The page below is a sample from the LabCE course Introduction to the ABO Blood Group System. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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"A" Antigenic Activity

Without H substance (also known as H antigen or substance H), there is no way for additional sugar attachment to take place. Additional sugar attachment is necessary for the development of A and B antigens. Therefore, without substance H there is no development of A and B antigens. Once substance H is developed, the addition of the sugar N-acetylgalactosamine to the terminal position of the chain gives the molecule "A" antigenic activity.