Anti-PF4 IgG light chains determine VITT antibodies. (A) Individual-expressing V3-21 antibody variable segments in their λ light chains in their peripheral B-cell pool may be more prone to develop PF4-specific antibody responses upon vaccination with viral vector-based vaccines, whereas (B) individuals not expressing V3-21 antibody variable segments within their B cell pool may be less prone to develop viral vector-induced thrombocytopenias. The figure was created with Biorender.