Electron microscopic view of the blood PMNs that have been cytochemically reacted for MPO and immunolabeled for lactoferrin.

(A) Patient PMNs display normal primary electron dense granules that have been opacified by the cytochemical MPO reaction and appear dark gray (pg); secondary lactoferrin-containing granules (sg) (identified by immunogold particles) are decreased in number and look larger than normal. (B) Control PMNs contain numerous lactoferrin-positive secondary granules (sg), as well as primary dark MPO⁺granules (pg). (Panels A and B, original magnification × 31 000; insets, × 4600.)