The mechanical characteristics of the structural components of polymorphonuclear granulocytes (PMN) and lymphocytes were tested by aspirating tongues from these cells into micropipettes with internal diameter (Dp)
2um. PMN tongues deformed in a continuous manner, while lymphocyte deformation was slower and approached a limiting steady level. The greater rigidity of the lymphocytes appeared to arise from their large, solid nucleus, while the PMN cytoplasm acted as a viscous fluid at these large scale deformations. Overall WEC deformability was also tested by measuring the time required for complete cell entry (te) into pipettes with Dp = 4 to 6um. The relative ease of entry of PMN and lymphocytes depended on the pipette diameter; lymphocytes entered more slowly than PMN for Dp = 4um, but more quickly than PMN for Dp = 5.9um. The predicted threshold pressure at which entry could just be induced was greater for lymphocytes than for PMN at Dp < 5um, but similar for the two cell types for Dp
5um. Apparently the greater stiffness of lymphocytes makes it relatively difficult for them to enter the smaller pipettes, but for the larger pipettes their smaller volume allows them to enter more rapidly than the PMN. Thus, the relative contributions of PMN and lymphocytes to blood flow resistance are likely to depend strongly on the vessel size. It was also noted that active PMN with pseudopodia took much longer to enter pipettes than smooth, passive PMN, therefore indicating that rheological tests on white blood cells must consider the possibility of cell activation.
