Looking Beyond the Cutaneous Manifestations of Covid 19, Part 2: The Pathology and Pathogenesis - A Review

This is the second part of the article, titled "looking beyond the cutaneous manifestations of Covid 19 Part 1: The Clinical Spectrum – A Review”, which is exclusively relegated to the pathology and pathogenesis aspects. The cutaneous manifestations of Covid 19 are classified into four broad groups, from the pathology and pathogenesis point of view and the histopathology of all the cutaneous lesions are briefly reviewed. The role of vasculitis and endothelitis in the pathogenesis of skin lesions in Covid 19, are discussed. The vasculitis and thrombotic microangiopathy (TMA) are discussed at length as they occupy the centre stage of pathogenesis, in the literature, at present. The various types of vasculitis reported in literature, are classified on the basis of skin lesions seen and the rationality of the various terms used in the context of the pathogenesis are explained. It is stressed that that the central players in the pathogenesis of skin lesions in Covid 19 are, vasculitis, activation of complement pathways and coagulation cascade and the cross- talk between the two at various points in their respective pathways. For the details of the complement activation, activation of coagulative cascade and for the details of the role of innate and adaptive immunity system, the readers may refer to the author’s previous article titled “Local Immunity Concept in the Context of the Novel Corona Viral Infection- A Consideration.” *** (reference of which is provided under the additional information at the end of the article). Some contentious issues concerning the role of the vasculitis and immune complex mediated damage, of the vessel wall in the pathogenesis of cutaneous lesions of Covid 19, are discussed. A non-immune, non-vasculitis, alternative mechanism is suggested by floating a two hit hypothesis. The necessity to apply the rigours of diagnostic criteria of vasculitis is emphasized to get a standard picture of the histopathology and pathogenesis of cutaneous manifestations of Covid 19 by future research.