Studies were made of the filterability of normal and abnormal red cells through an inert membrane-type (Millipore) filter having uniform non-branching capillary pores 5 µ in diameter. With this filter it was possible to remove selectively certain kinds of abnormal red cells from mixed suspensions of normal and abnormal red cells.

The red cells of patients with sickle cell anemia were selectively trapped by the filter under low oxygen tensions. Such sickled red cells were unable to pass through the filter even under a pressure differential across the filter (perfusion pressure) of over 120 mm. Hg.

Red cells agglutinated by anti-A were completely retained by the filter at perfusing pressures of 4 mm. Hg., but were partially disagglutinated and forced through the filter when the pressure was increased. Red cells coated with an incomplete (Rh) agglutinin and red cells from a patient with acquired hemolytic anemia were retained by the filter only in the presence of rouleaux-producing agents such as P.V.P. or fibrinogen and only at low perfusion pressures.

A small fraction of the red cells of patients with hereditary spherocytosis were trapped by a single passage through the filter; the trapped cells were largely derived from the minor population of hyperspheroidal cells.

The relevance of these findings to the mechanisms of red cell sequestration in vivo is discussed.

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