High Free IgE and Mast Cell Activation in Long COVID: Mechanisms of Persistent Immune Dysregulation

Background: Elevated serum IgE has been reported in severe COVID-19, suggesting that mast cell activation, allergic-like responses, and possible viral immune evasion occur. Objective: This study aimed to assess serum IgE, IgG, eosinophils, basophils, IL-10, and IL-33 in COVID-19 patients, and evaluate the infiltration of mast cells, basophils, and plasma cells in fatal cases. Methods: This retrospective study included 21 patients with severe COVID-19 or related respiratory conditions hospitalized in Plovdiv, Bulgaria (February 2020–May 2022). Serum immunoglobulins were quantified via immunoassays; IL-10 and IL-33 were also measured. Lung tissues from 30 autopsies were examined histologically and immunohistochemically using CD117 (mast cells) and CD138 (plasma cells). Results: Elevated IgE (>100 IU/mL) occurred in 10/21 patients, with two patients exhibiting levels exceeding 1000 IU/mL. High IgE correlated with reduced eosinophils and basophils, except in post-COVID lobar pneumonia. IL-10 was significantly increased, while IL-33 was reduced in acute and long COVID. Lung histology showed the accumulation of mast cells and plasma cells (5–20/field) during the diffuse alveolar damage and acute respiratory distress syndrome (ARDS) phases, but not in later fibrotic stages. Basophils are located near capillary basement membranes and the endothelium. Conclusions: SARS-CoV-2 may induce IgE-driven allergic-like mechanisms that contribute to severity. Monitoring IgE and mast cell activity may provide prognostic and therapeutic value, while elevated IgG4 could mitigate the effects of IgE.