Human milk is a living biological tissue which contains a microbiome, host-derived cells (alive and dead), and myriad bioactive factors

Why human milk is a living biological tissue

Tissues are collections of cells and extracellular products which perform specific functions. Scientists and health authorities describe milk as a 'secretion of a human tissue', that is, a biological fluid secreted by the mammary gland, and also as a living tissue in itself because it contains viable cellular components.

In regulatory and ethical discussions, especially concerning milk banking and donation, human milk is treated similarly to other human tissues like blood or plasma. It is donated, screened, and distributed under tissue banking principles.

The living components of human milk

Maternal cells

Epithelial cells: from the mammary gland lining
Immune cells: macrophages, neutrophils, lymphocytes (T cells, B cells, NK cells). Mothers of exclusively breastfed infants have higher baseline leucocyte counts in their breast milk than those who are not exclusively feeding.
Stem/progenitor cells: with potential to differentiate (still under active research)

Microbes (the human milk microbiome)

Viable bacteria: e.g., Streptococcus, Staphylococcus, Lactobacillus, Bifidobacterium
Fungi and viruses (in much smaller numbers), archaea, protozoa

Bioactive factors in human milk

The rest of milk (nutrients, antibodies, hormones, dead cells, DNA/RNA fragments) is bioactive, but not "alive". This includes extracellular vesicles, (exosomes and microvesicles), which

Are not “cells” but contain functional RNA, proteins, and enzymes that are biologically active
Carry living signals which influence infant cells.

Human milk contains not just living cells but also dead cells and cell fragments

Microbiome-derived cells and cell fragments

Human milk also contains non-viable microbes and microbial debris, including dead microbial cells, cell wall fragments, DNA, RNA, and metabolites.
These dead cells still influence the infant by stimulating immune development and tolerance.

Host-derived cells and cell fragments

Milk also contains apoptotic (or dead) mammary epithelial cells which are part of the normal cell turnover. These exfoliated, dead cells may still continue to secrete milk components for a time.