The primary inflammatory events occurring in the adipose tissue (AT) during high fat diet (HFD)-induced obesity are poorly defined. The present study was undertaken to characterize, in wild-type(+/+) and lymphocyte deficient RAG2(-/-) mice under HFD, the changes in AT immune cells by flow cytometry analyses. In (+/+) mice, early accumulation of AT B-cells was observed. followed by increased AT T-cell numbers and finally by the appearance of insulin resistance and AT macrophage accumulation. Lack of lymphocytes in the RAG2(-/-) mice did not affect the onset of obesity and the state of insulin resistance. However, a striking accumulation of AT NK cells and activated macrophages was detected. The present study demonstrates that AT is the site of an unexpected dynamic in innate and adaptive cells during diet-induced obesity and insulin resistance. Moreover it appears that early AT lymphocyte infiltration Could be considered a protective process to temper adipose tissue inflammation. (C) 2009 Elsevier Inc. All rights reserved.