A plasma sample containing anti-D is tested against 4 differ…

A plasma sample containing anti-D is tested against 4 different Rh(D) positive RBCs at the 37 C phase of testing with the following results: Cell 1 = 3+Cell 2 = 2+Cell 3 = 1+Cell 4 = 4+What accounts for the different strengths of reactivity seen with this antibody?