C4A and COVID-19: Combined log2 expression of the three proteins C4a, CD14 and NRCAM (y = (C4A + CD14)/2 − NRCAM) used in a simple linear fit conducted on all COVID-19 samples resulted in dependence with a rather high coefficient of determination, R2 = 0.67 (see Additional file 1: Figure S1), meaning that these distinct proteins and their pathways are only activated in COVID-19 with BSI.