Hi Byungkuk. PBMCs—isolated from peripheral blood—should generally have the same composition. When obtained from nonperipheral sites, I believe they may have varying compositions and phenotype.

Successfully only across postcapillary venules of tissues, the leukocyte transmigration is the culmination overlapping steps mediated by multiple ICAMs molecules expressed normally on luminal surface in endothelial cells of postcapillary veins; but exactlly to lung is the final road without comeback of sequential pools of leukocytes,PBMC that were starting in bone marrow (pool of production and pool of maturation), passing to blood (pool vascular), arrived to capillars (pool marginal) and arrived to tissues (pool tisular) (in lung is intersticium and alveolar lumen), where before monocytes and now macrophagues don´t comeback to pool vascular, aspect that explain normal variation in counts cells of PBMC between sites of question.

Consult like example:

Evidence of a Selective Major Vascular Marginal Pool of Lymphocytes in the Lung. Reinhard Pabst , Richard M. Binns , Stephen T. Licence , and Marita Peter. https://doi.org/10.1164/ajrccm/136.5.1213 PubMed: 3674582

https://www.researchgate.net/publication/19486517_Evidence_of_a_Selective_Major_Vascular_Marginal_Pool_of_Lymphocytes_in_the_Lung

Aditionally, between heart and facial vein, other important aspect that consider numerical desviations cell counts are changes by of Cardiac output according anatomical positions Prone or Supine; Consult "West lung zones" that explain variations mathematical relationship Ventilation and perfusion (V/Q).

PULMONARY PERFUSION IN THE PRONE AND SUPINE POSTURES IN THE NORMAL HUMAN LUNG G. Kim Prisk,1,2 Kei Yamada,3 A. Cortney Henderson,1 Tatsuya J. Arai,1 David L. Levin,2,* Richard B. Buxton,2 and Susan R. Hopkins1,2 J Appl Physiol. Author manuscript; available in PMC 2008 Sep 1. J Appl Physiol. 2007 Sep; 103(3): 883–894. Published online 2007 Jun 14. doi: 10.1152/japplphysiol.00292.2007

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2399900/

Department of Medicine, University of California, San Diego, La Jolla CA, 92093

2 Department of Radiology, University of California, San Diego, La Jolla CA, 92093

3 School of Medicine, University of California, San Diego, La Jolla CA, 92093

Address for Correspondence: G. Kim Prisk Ph.D., D.Sc., Department of Medicine, 0931, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0931, Phone 858-455-4756, FAX 858-455-4765, ude.dscu@ksirpk

*Current address for D.L. Levin: Department of Radiology, Mayo Clinic, Rochester, MN 55905