The origin of the polymorphonuclear leucocytes found in the intermediary and subcapsular sinuses of the popliteal lymph node during acute bacterial lymphadenitis, and the effect of this leucocyte infiltration on the passage of bacteria through the lymph node have been investigated. It has been demonstrated that:
1. The polymorphonuclear leucocytes in the nodal sinuses originate both from blood vessels of the lymph node and from the primary inflammatory focus in the tissues.
2. Granulocytes invading the intermediary sinuses of the infected lymph node arise primarily from capillaries lining these sinuses.
3. Most of the polymorphonuclear leucocytes in the subcapsular sinus, on the other hand, originate from the inflammatory focus in the tissues and appear to traverse the node by way of this peripheral sinus.
4. The bacteremia following direct intralymphatic injection of pneumococci is suppressed by the presence of preformed inflammatory exudate in the nodal sinuses indicating that the filtering capacity of the node is thereby greatly increased.