C4A and COVID-19: Some of the proteins specifically decreasingly or increasingly expressed, which are represented by different colors, are related to COVID-19 or acute respiratory disorder in general, as shown in Fig. 2C. Of these, the expression of apolipoprotein A-I (APOA1), alpha-2-HS-glycoprotein (AHSG), hemoglobin beta chain (HBB), serum amyloid A-4 (SAA4), alpha-1-acid glycoprotein 1 (ORM1), and complement C4A (C4A) was found to be significantly reduced in the NcNs group compared to the NcSus group.