Antibody responses in COVID-19: clones specific to the spike protein RBD antibodies develop in most patients in response to COVID-19 infection. A subset of these overlap with cross-reactivity to PF4/heparin antibodies as seen in HIT. The B-cell clones that express these antibodies contain HCDR3 regions with motifs seen in HIT such as “RKH” and “Y5.” Some of these antibodies can activate platelets in a modified assay (PEA^CpG) and may participate in the immunothrombosis-driven coagulopathy that is associated with severe COVID-19 infection. Servier Medical Art Creative Commons Attribution 3.0 France.