Is there evidence of immune activation or inflammation in tissues despite normal blood tests in long COVID?

Yes. Multiple studies show immune activation and inflammation in tissues despite normal routine blood tests in long COVID.

A 2026 study of colorectal tissue from 44 long COVID patients and 13 recovered controls found 4,757 differentially expressed genes in the tissue, with downregulated pathways for MHC class I antigen processing, lysosome activity, and phagocytosis. Yet matched blood samples showed only 3 differentially expressed genes in PBMCs and no significant plasma proteomic differences [2]. The tissue inflammation was invisible to standard blood work.

A 2025 study of 21 severely ill COVID-19 patients found elevated serum IgE and mast cell activation, with autopsy lung tissue showing 5-20 mast cells per high-power field and clusters of basophils and plasma cells in ARDS stages [1]. Another 2025 study in hamsters showed that SARS-CoV-2 infection triggered innate immune activation and mitochondrial ROS that persistently disrupted the cardiac conduction system, with macrophage infiltration concentrated in the AV bundle (0.37 vs 0.09 in ventricle, p<0.0001) even after the virus was cleared from heart tissue [4].

A 2026 study of 16 post-COVID patients with respiratory lung abnormalities found a monocyte-macrophage-fibroblast axis in the lung, with profibrotic SPP1+ alveolar macrophages enriched in bronchoalveolar lavage fluid. That fluid, when applied to human lung fibroblasts, increased proliferation by 39% (p<0.001) [3]. A 2023 study found that long COVID patients with persistent dyspnea had reduced capillary blood volume and increased sDLNO/sDLCO ratio, indicating a capillary-blood-volume disorder that spirometry and CT scans miss entirely [5].

The mechanism is clear: the immune activation is compartmentalized to tissues - lung, heart, gut, olfactory epithelium - while the blood compartment appears quiet. Routine blood tests measure the systemic circulation, not the local tissue environment. This is why standard labs often come back normal while patients remain symptomatic.

My call: tissue-level immune activation and inflammation are well-documented in long COVID despite normal routine blood tests. The evidence is strong and consistent across multiple tissue types and independent research groups.