Juxtacrine signaling by PAF mediates PMN activation. PAF displayed on the plasma membranes of inflamed endothelial cells delivers an activating signal to PMNs that are tethered to the endothelial surface by P-selectin (9). This signal, mediated by the PAF receptor, primes PMNs for enhanced granular secretion and oxygen radical generation, among other inflammatory responses (3). Under normal regulation, this multistep adhesion and signaling pathway contributes to host defense. The same molecular events, when dysregulated or uncontrolled, can induce ALI and other syndromes of inflammatory tissue injury.