The lymphocytosis is due to an accumulation of B lymphocytes expressing CD5 and /or CD11b. A. PBMCs were isolated from BLV wild type infected sheep (n° 4535) and labeled with anti-sIgM 1H4 monoclonal in combination with CD5 or CD11b antibodies. Ten thousand cells analyzed by flow cytometry are represented as dot plots (Y axis = B cells; X axis = CD5 or CD11b expressing cells). Illustrated data correspond to the dot plots performed at day 0 (provirus injection) and at the seroconversion day. The total numbers of B cells are indicated in the upper quadrants. B. Histogram representation of the absolute numbers of B cells expressing (right panels) or not (left panels) CD5 or CD11b in sheep infected with wild type viruses (n° 4535 and 4536), or mutants (pBLVIG4 in n° 4537 / 4538, pBLVCRX3 in n° 4539 / 4541, pBLVCRE3X in n° 4542 / 4543, pBLVA60V in n° 4544 / 4545). Sheep n° 4533 and 4534 were used as uninfected controls. * represents the absolute number of cells at day 24 of the experiment.