Group B: the W1:2-3 loop is essential for PA anti-αIIbβ3 Ab binding. (A) Relative binding of PA Abs in 5 patients (PTs 3, 12, 36, 41 and 42) to m(W1:1-2)H (black), m(W1:2-3)H (white), and Hm(W1:2-3)H (shaded) compared with wt αIIbβ3. (B) Relative binding of PA Abs to mouse αIIb replaced from the N-terminus to the indicated loops with the corresponding human sequences. (C) Relative binding of PA Abs to human αIIb replaced the indicated loop with the corresponding mouse sequences. (D) Relative binding of PA Abs to mH(W1:2-3)m (black), mH(W1:2-3–W3:3-4)m (white), and mH(W1:2-3–W3:4-1)m (shaded). (E) Relative binding of PA Abs to T42A (black), G44N (white), and P45A (shaded) mutants. (F) Relative binding of PA Abs to P135L (black), E136G (white), and R139G (shaded) mutants. Shown were means of ≥ 2 independent experiments.