Group A: the W1:1-2 and W2:3-4 loops are essential for PA anti-αIIbβ3 Ab binding. (A) Relative binding of PA Abs in 2 patients (PTs 17 and 23) to m(W1:4-1)H (black), m(W1:1-2)H (white), and Hm(W1:1-2)H (shaded) compared with wt αIIbβ3. (B) Relative binding of PA Abs to H(W2:2-3)m (black), H(W2:3-4)m (white), and Hm(W2:3-4)H (shaded). (C) Relative binding of PA Abs to mH(W1:1-2–W2:3-4)m that the mouse αIIb carried the human W1:1-2 to W2:3-4 sequences. (D) Relative binding of PA Abs to S29K (black) and R32S (white) mutants. (E) Relative binding of PA Abs to P135L (black), E136Q (white) and R139G (shaded) mutants. Shown were means of ≥ 2 independent experiments.