Fig. 1.

Immune responses and protection against SARS-CoV-2: (i, ii, iii) SARS-CoV-2 binds to the ACE-2 receptor and gets internalized via endocytosis with the help of TMPRSS2. (iv) Inside the host cell Viral RNA is recognized by TLR3 and some sensors of viral infection (MDA 5 and RIG- I), (v) which induce production of IFN-I, and (vi) activation of various cellular responses (ROS production, calcium influx), (vii) leading to the production and activation of NLRp3 inflammasome, which further activates caspases that cleaves pro-IL-1β, pro-IL-18, and gasdermin D, leading to their activation (IL-1β, IL-18, and gasdermin D). (viii, ix) Macrophages also produce cytokines that activate enough amount of NK cells, (x, xi) leading to the production of IFNγ, IL-2, and granzyme B, which causes pyroptosis of the infected cell. (xii) However, the viral RNA can also use the host machinery to form new virions, (xiii) which can be recognized by the APCs leading to their presentation to the T cells, which activates Th and Tc cells. (xiv) Th cells produce IFNγ, IL-2, and TNF α, (xv, xvi) which activate the B cells to produce spike-specific and neutralizing antibodies resulting in the virus clearance. (xvii) The cytokines produced from Th cells also induce the Tc cells to produce perforins, (xviii) which leads to pyroptosis of the infected cell and protection against the disease.