Group C: the W3:4-1 loop is essential for PA anti-αIIbβ3 Ab binding. (A) Relative binding of PA Abs to m(W2:3-4)H (black), m(W3:4-1)H (white), and Hm(W3:4-1)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 human corresponding sequences. (C) Relative binding of PA Abs to human αIIb replaced the indicated loop with the mouse corresponding sequences. (D) Relative binding of PA Abs to mH(W3:4-1–W4:1-2)m and mH(W3:4-1–β-propeller)m that the mouse αIIb carried the human sequences from W3:4-1 to the C-terminal of the β-propeller domain. (E) Relative binding of PA Abs to 2 amino acids insertion mutant in W3:4-1 loop (KO variant αIIbβ3). Shown were means of ≥ 2 independent experiments.