The larger the number of antigen sites, the more antibody will attach to the cells, thus increasing the chances of cross-bridges forming. Thus, IgG anti-D cannot agglutinate the normal Rh positive cells, which have 10,000 - 30,000 D antigen per cell, but can agglutinate rare D-- cells which have about 500,000 D sites per cell.
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| Number of Antigenic Sites on the Red Cell |