Since December 2019, COVID-19 disease caused by SARS-CoV-2 virus has spread world-wide as pandemic causing large number of infected persons and lot of deaths of COVID patients. There is currently no definite medicine for treatment of the disease as well as any preventive measure. With the progress of multiplication of viruses in the human, activation and amplification of host immune response take place causing massive release of varieties of cytokines like IL-1, IL-6, IL-17 etc and different t types of IFNs , TNF to form Cytokine Storm. From studies of COVID-19 patients in hospital, it has also been noted that uncontrolled inflammation due to elevated levels of cytokine (cytokine storm) is responsible for causing severity of the disease. In this article the properties of cytokine, types of cytokine and other details have been discussed. The mechanism of signalling of cytokine to immunity and the reason for causing cytokine storm with its role in causing fatalities in COVID 19 patients have also been pointed out. Low infection of virus persists in upper airway that causes similar to seasonal respiratory illness but with severe infections virus remains in the lower respiratory tract leading to lung injury, Acute Respiratory Distress Syndrome (ARDS) and sometimes death. The role of cytokine storm in causing severity of disease has been discussed.

Keywords: Cytokine, IL-6, IL-17, IL-33 families, Signalling mechanism, Immunity, Cytokine Storm, Role in COVID-19.