The lung capillaries contain marginated intravascular B cells. (A) Flow cytometry compared B cells (fluorescently conjugated anti-CD19 mAb) within a whole-lung homogenate with BAL fluid. SSC, side scatter. (B and C) Representative flow cytometry and quantification of intravascular (CD19⁺/B220⁺) versus extravascular (CD19⁺/B220⁻) B cells using a two-antibody staining technique (A–C, n = 3 independent experiments with six mice total). (D) Lung intravital microscopy visualized both intravascular neutrophils (fluorescently conjugated anti-Ly6G mAb) and B cells (Cd19^ZsGreen1). Scale bar, 70 µm. (E and F) The transit times of neutrophils and B cells were quantified using intravital microscopy. Marginated cells were observed in the FOV for up to 600 s. Cells at time point 0 were followed for their duration in the lung vasculature and expressed as a percentage of the initial total B cell numbers in each FOV (D–F, n = 4 independent experiments using four mice total; each dot represents one FOV). Pooled data are presented as mean ± SD. Exact P values were determined using Student’s t test. ****, P < 0.0001.