Neutrophilia, lymphopenia and myeloid dysfunction: a living review of the quantitative changes to innate and adaptive immune cells which define COVID-19 pathology
Authors: Codd Amy S , Hanna Stephanie J , Compeer Ewoud B , Richter Felix C , Pring Eleanor J , Gea-Mallorquí Ester , Borsa Mariana , Moon Owen R , Scourfield D Oliver ,   Oxford-Cardiff COVID-19 Literature Consortium, Ahern David J , Almuttaqi Hannah , Alonzi Dominic S , Alrubayyi Aljawharah , Alsaleh Ghada , Bart Valentina M T , Batchelor Vicky , Bayliss Rebecca , Berthold Dorothée L , Bezbradica Jelena S , Bharuchq Tehmina , Borrmann Helene , Borsa Mariana , Borst Rowie , Brun Juliane , Burnell Stephanie , Capitani Lorenzo , Cavounidis Athena , Chapman Lucy , Chauveau Anne , Cifuentes Liliana , Codd Amy Susan , Compeer Ewoud Bernardus , Coveney Clarissa , Cross Amy , Danielli Sara , Davies Luke C , Dendrou Calliope A , Dimonte Sandra , Durairaj Ruban Rex Peter , Dustin Lynn B , Dyer Arthur , Fielding Ceri , Fischer Fabian , Gallimore Awen , Galloway Sarah , Gammage Anís , Gea-Mallorquí Ester , Godkin Andrew , Hanna Stephanie Jean , Heuberger Cornelia , Hulin-Curtis Sarah , Issa Fadi , Jones Emma , Jones Ruth , Ladell Kristin , Lauder Sarah N , Liddiard Kate , Ligoxygakis Petros , Lu Fangfang , MacLachlan Bruce , Maleki-Toyserkani Shayda , Mann Elizabeth H , Marzeda Anna M , Matthews Reginald James , Mazet Julie M , Milicic Anita , Mitchell Emma , Moon Owen , Nguyen Van Dien , O'Hanlon Miriam , Pavillet Clara Eléonore , Peppa Dimitra , Pires Ana , Pring Eleanor , Quastel Max , Reed Sophie , Rehwinkel Jan , Richmond Niamh , Richter Felix Clemens , Robinson Alice J B , Rodrigues Patrícia R S , Sabberwal Pragati , Sami Arvind , Peres Raphael Sanches , Sattentau Quentin , Schonfeldova Barbora , Scourfield David Oliver , Selvakumar Tharini A , Shepherd Freya R , Shorten Cariad , Simon Anna Katharina , Smith Adrian L , Crespo Alicia Teijeira , Tellier Michael , Thornton Emily , Uhl Lion F K , van Grinsven Erinke , Wann Angus K T , Williams Richard , Wilson Joseph D , Zhou Dingxi , Zhu Zihan , Gallimore Awen M , Milicic Anita 
Published: July 2021

Abstract
Destabilization of balanced immune cell numbers and frequencies is a common feature of viral infections. This occurs due to, and further enhances, viral immune evasion and survival. Since the discovery of the Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2), which manifests in coronavirus disease 2019 (COVID-19), a great number of studies have described the association between this virus and pathologically increased or decreased immune cell counts. In this review, we consider the absolute and relative changes to innate and adaptive immune cell numbers, in COVID-19. In severe disease particularly, neutrophils are increased, which can lead to inflammation and tissue damage. Dysregulation of other granulocytes, basophils and eosinophils represents an unusual COVID-19 phenomenon. Contrastingly, the impact on the different types of monocytes leans more strongly to an altered phenotype, e.g. HLA-DR expression, rather than numerical changes. However, it is the adaptive immune response that bears the most profound impact of SARS-CoV-2 infection. T cell lymphopenia correlates with increased risk of intensive care unit admission and death; therefore, this parameter is particularly important for clinical decision-making. Mild and severe diseases differ in the rate of immune cell counts returning to normal levels post disease. Tracking the recovery trajectories of various immune cell counts may also have implications for long-term COVID-19 monitoring. This review represents a snapshot of our current knowledge, showing that much has been achieved in a short period of time. Alterations in counts of distinct immune cells represent an accessible metric to inform patient care decisions or predict disease outcomes.