Multisystem inflammatory syndrome in children (MIS-C) is a life-threatening disease occurring several weeks after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Deep immune profiling showed acute MIS-C patients had highly activated neutrophils, classical monocytes and memory CD8+ T-cells; increased frequencies of B-cell plasmablasts and double-negative B-cells. Post treatment samples from the same patients, taken when symptoms were resolving, identified recovery-associated immune features including CD163+ monocytes, emergence of a new population of immature neutrophils and, in some patients, a transient increase in arginase. Plasma profiling identified multiple features shared by MIS-C, Kawasaki Disease and COVID-19 and that therapeutic inhibition of IL6 may be preferable to IL1 or TNF-a. We identified potential new mechanisms of action for IVIG, the most commonly used drug to treat MIS-C. Finally, we showed systemic complement activation with high plasma C5b-9 levels is common in MIS-C, suggesting complement inhibitors could be used to treat the disease. Funding Information: Birmingham Women’s and Children’s Hospital Charity funded the single cell RNA sequencing analysis. No other external funding was received. Declaration of Interests: None declared. Ethics Approval Statement: Reviewed and approved by South of Birmingham Research Ethics Committee (REC: 17/WM/0453, IRAS: 233593).