KLRD1 and COVID-19: Based on the raw number of related terms in the enrichment, eight key genes can be identified—C1QB, C1QA, ISG15, SERPING1, VSIG4, KLRD1, TRPM4, and HFE. Four of these genes are related to both groups 2 and 3 and to the terms “complement activation” and “negative regulation of immune response,” as well as the aforementioned “humoral response”---C1QA, C1QB, SERPING1, and VSIG4. Going further, we would like to focus our attention on these four genes and on their role and the wider role of the complement system in COVID-19.