Analysis of peripheral blood IgG⁺B-cell repertoires in patients with HIT (HIT), patients given heparin without experiencing HIT (non-HIT), and healthy persons. (A) Percentage of IgG⁺ B cells using the J_H6 gene in the 3 populations. (B) Percentage of IgG⁺ B cells using a V_H3-J_H6 combination. (C) Percentage of IgG⁺ B cells possessing at least 5 Tyr residues in HCDR3. (D) Percentage of IgG⁺ B cells possessing an RKH or Y₅ HCDR3 motif. Sequences identified in the 3 groups were pooled for analysis. (E) HCDR3 sequences containing RKH or Y₅ identified in repertoire sequencing of peripheral blood IgG⁺ B cells from patients with HIT that were expressed as recombinant IgG antibodies in a pairing with κ chains containing the indicated κ chain CDR3s selected, as described in text. (F) Functional behavior of 9 clones containing HCDR3 and LCDR3 sequences, shown in Figure 5E. Left: Each of 9 clones reacted strongly with PF4/H in ELISA. Right: Two clones, 1 with RKH and 1 with the Y₅ motif, also activated PF4-treated platelets. P values were calculated by Fisher exact test. ∗P < .05, ∗∗P < .01, ∗∗∗P < .001, and ∗∗∗∗P < .0001.