Mammary glandular tissues and mammary secretions were obtained from sheep at 2–60 d after weaning to
study the leucocyte phenotypes associated with mammary involution. From 2–4 d after weaning, neutrophils
were the predominant leucocytes in the alveolar and ductal lumina. Lymphocytes were present in the
alveolar and ductal epithelium, interalveolar and periductal areas. Most of the lymphocytes in the alveolar
and ductal epithelium (IEL) were CD8+, some were CD45R+ and few were CD4+. In the periductal clusters
and in the interalveolar areas most of the lymphocytes were CD4+. There was a significant increase
(P < 0.05) in the percentages of CD45R+ granulated IEL from 2 to 7 d after weaning, and this paralleled the
increase in the percentages of apoptotic cells in the glandular epithelium. By 7–60 d after weaning, most cells
within the alveolar and ductal lumina were macrophages followed by predominantly CD8+ lymphocytes.
CD8+ lymphocytes were still predominant in the alveolar and ductal epithelium while CD4+ cells were
predominant in the interalveolar areas. Very few γδ+ T cells were observed at all the stages examined. The
cells in the mammary secretions correlated with those observed in the alveolar and ductal lumina. At the
early stages of involution, the neutrophils and macrophages were heavily laden with lipid droplets, casein
and cellular debris. The most interesting feature was the presence of cells either with extensive cytoplasmic
processes (LCA+ MHC class II+) or cytoplasmic veils (LCA+ MHC class II+CD1+), probably dendritic cells. It
is concluded that the cellular constituents of the mammary gland at the latter part of involution may afford
the mammary gland more resistance to infection than the lactating gland and the gland at early stages of
involution. The CD45R+ IEL may trigger apoptotic cell death in the mammary glandular epithelium during
mammary involution.