Mucosal immunity, including secretory IgA (sIgA), plays an important role in early defense against respiratory pathogens. Saliva analysis, the most convenient way to measure sIgA, is used to characterize the mucosal immune response to many viral infections including SARS, MERS, influenza, HIV, and RSV. The study determined that mucosal immunity, including secretory IgA, plays an important role in host defense against respiratory pathogens, and their data suggest that this may occur in COVID-19. This monograph presents the results of a study of lysozyme activity and sIgA concentrations in Covid-19 patients. The monograph is prepared for laboratory physicians, general practitioners, students and masters of medical universities.