The course and severity of COVID-19 in individual patients is largely influenced by the interaction between the SARS-CoV-2 coronavirus and the human immune system. Normally, the antiviral immune response of natural killer cells (NK cells) is an important step in combating viral replication in the early phase of the infection. On their surface, these killer cells have special, activating receptors, including the NKG2C receptor, which communicates with an infected cell via one of its specialized surface structures, HLA-E. This interaction results in the destruction of virus-infected cells. However, due to a genetic variation, approximately 4% of the population naturally lack the activation receptor NKG2C, and in 30% of the population this receptor is only partially available.