Lymphatic clotting is widespread in LDLNs of COVID-19 decedents. (A) Representative immunofluorescence image showing a large lymphatic clot in an LDLN section with fibrin (green) and podoplanin (red), with inset (Ai) highlighting the fibrillar structure of the fibrin clot in 488 autofluorescence (gray). (B-D) Representative images showing intralymphatic fibrin clots in LDLNs of (B) COVID-19 decedents, (C) control decedents without viral infections, and (D) H1N1 decedents, with arrows indicating partially or fully clotted vessels and arrowheads indicating open vessels. Scale bars in panels A-D, 100 μm. (E) Lymphatic clotting score rubric showing examples for each score: 0, no fibrin (fully open); 1, fibrin confined to endothelium; 2, minimal intralymphatic fibrin (<20% of vessel lumen); 3, partially occluded (>20% of lumen); and 4, fully occluded. (F) Percentage of clotted lymphatic vessels (with a fibrin score of 3-4) in LDLNs from COVID-19, control, and H1N1 decedents; each dot shows the average from 3 sections of each patient. (G) Average lymphatic fibrin score in each LDLN. Bars in panels F-G represent median ±95% confidence interval; ∗P < .05, ∗∗P < .01 by one-way ANOVA with Tukey multiple comparisons post-test.