BACKGROUND:Tissue recruitment of eosinophils and activated lymphocytes is a characteristic feature of allergic reactions. However, little is known about the involvement of specific adhesion molecules in the traffic of leucocytes during the allergic response. OBJECTIVE:To use a sheep mammary infusion model to characterize the kinetics of cell recruitment and expression of cellular adhesion molecules and activation markers on eosinophils and lymphocytes involved in an allergic-type response. METHODS:Mature non-lactating ewes were primed and challenged by direct infusion of the mammary glands with nematode larvae. Using a non-invasive method of saline infusion and 'milking' of the glands, large numbers of inflammatory cells were repeatedly sampled over 10 to 96 h following their migration into the mammary gland lumen, and analyzed by 2-colour flow cytometry. RESULTS:Leucocyte recruitment into the mammary lumen was characterized by two separate phases involving an acute neutrophilic response at 10 h post-challenge, followed by a dramatic reduction in neutrophils and appearance of eosinophils and activated lymphocytes. From 48 h post-challenge, eosinophils were predominant and represented 40 to 65% of leucocytes in the mammary lavage (MAL). Increases in activated CD4+ T cells and gammadelta+ T cells were also observed at this time-point. The kinetics of expression of cell surface molecules on eosinophils and lymphocytes in blood and MAL were compared during the course of the allergic-type reaction. Adhesion molecule expression on lymphocytes was modulated following allergen challenge and an activation of MAL vs. blood lymphocytes was seen during the later stages of the allergic response. Eosinophil expression of VLA-4 and l-selectin was down-regulated compared with blood at all time-points examined. There were high levels of expression of CD11b and CD44 on eosinophils during the early compared to the late-phase of the allergic reaction. CONCLUSION:These results indicate the existence of two separate mechanisms of eosinophil recruitment during the allergic inflammatory response.