ABSTRACT
In general, B and T lymphocytes, which are involved in adaptive immunity, are in charge of cell-mediated response and antibody-mediated immunity, respectively. Another subset of lymphocytes, known as natural killer (NK) cells, are innate effector cells. They serve as the body's initial line of defence against viral infections. They perform the task of eliminating stressed cells and are crucial for tumour immunity. These cells are capable of performing their killing function without clonal expansion and differentiation following activation. The NK cells will immediately eliminate infected host cells but other lymphocytes need lymphocyte proliferative response which takes several days and further differentiate into effector cells, so that they eliminate host cells infected by the viral pathogen. The NK cells also form a bridge between the adaptive and innate immunity and play significant roles during respiratory infection. Number and the role of NK cells correlate with the severity of severe acute respiratory syndrome (SARS);the number and the percentage of CD158b+ NK cells in severe SARS infection were significantly less in number than those with mild cases. Innate defence mechanisms, particularly NK cells, are able to control SARS infection even in the absence of T cells and antibodies, according to cellular immunological responses to SARS infection in mice. As a result, NK cells are crucial in the fight against viral infections of the respiratory system. As an innate immune system, they serve as the initial line of virus protection. It is possible to do additional research to take advantage of this NK cell trait and develop a cutting-edge therapeutic approach to fight developing respiratory viral diseases.