The appearance of fluorescein-labelled lymphocytes in lymph following in vitro or in vivo labelling: the route of lymphocyte recirculation through mesenteric lymph nodes.

Fluorescein isothiocyanate (FITC) has been used to study lymphocyte migration in sheep. After being labelled in vitro with FITC, lymphocytes migrated from blood into lymph at the same rate and with the same recovery as lymphocytes labelled with with the radioisotope 51chromium. The in vivo labelling of mesenteric lymph nodes (MLN) with FITC resulted in high numbers of labelled lymphocytes appearing in prescapular lymph. However, the appearance of the FITC-labelled lymphocytes in the prescapular lymph could be prevented by cannulating the main intestinal lymph duct prior to the in vivo labelling procedure. It was concluded that lymphocytes labelled in vivo within the MLN required an intact lymphatic system to reach the blood circulation and did not enter the venous circulation directly from the MLN.