Positive CP-10 staining (A) of microvessels and capillaries (n = 3, 4 fields/footpad) was obtained in 18 of 110 vessels counted when footpads from sensitized mice were challenged with ovalbumin and sections were stained with affinity-purified anti–CP-10 IgG. Note the positive staining of the neutrophil within the vessel. No reactivity occurred with control rabbit IgG (B). Serial sections show CP-10–positive staining of a small blood vessel and PMN (C) with anti–CP-10, whereas the same vessel was negative (D) when reactivity was competed with rCP-10. Original magnification × 1,000.