CD40LG and respiratory tract infectious disorder: Surprisingly, some samples drawn before the COVID-19 pandemic revealed IgM binding to the S1+S2 ECD fragment, S1 subunit, and the S2 ECD (Figure 2(a)), likely reflecting IgM crossreactivity with similar epitopes on related β coronaviruses responsible for common human respiratory infections (e.g., OC43 and HKU1).