Contour plots show the number of PSGL-1 molecules represented by a MEMBRANE UNIT. The number of PSGL-1 represented are shown for two different LEUKOCYTE MEMBRANES. Each of the 600 MEMBRANE UNITS within each MEMBRANE contains one PSGL1 agent (see Figure 2C). Each PSGL1 TotalNumber parameter value (indicated by the scale on the right) specifies the number of PSGL-1 molecules represented in a specified MEMBRANE UNIT. When averaged over the entire MEMBRANE, a typical PSGL1 represents 150 ± 5 PSGL-1 (see Table 3). The 8 × 10 region outlined in white is the CONTACT ZONE. Arrows specify the row of MEMBRANE UNITS (at the "rear" of the CONTACT ZONE) that experiences RearForce, when the LEUKOCYTE is moving from right to left.