A milk-producing glandular epithelial cell that is part of a mammary gland alveolus and differentiates from a luminal adaptive secretory precursor cell during secretory differentiation (also termed lactogenesis I). Following secretory activation (also termed lactogenesis II), a lactocyte is involved in the synthesis and/or transport of milk constituents including proteins, oligosaccharides, lactose, micronutrients, fat, hormones, immunoglobulins, and cytokines into the lumen of the lactating mammary gland. [ http://www.ncbi.nlm.nih.gov/pubmed/19022771 http://www.ncbi.nlm.nih.gov/pubmed/35377806 http://www.ncbi.nlm.nih.gov/pubmed/35091553 GOC:tfm http://www.ncbi.nlm.nih.gov/pubmed/28717367 doi:10.1177/156482659601700404 ]