I agree that the reference said that 80% of the cells are macrophages, but
Hale & Hartmann's "Textbook of Human Lactation" gives a different picture.
In Chapter 3 by Berry, Piper, Thomas and Cregan, pg 41 onwards talks about
cells in breast milk. In short, breast milk contains several kinds of
cells including immune cells (leukocytes, or leucocytes in a different
chapter).
At birth, 99% of the cells in breast milk are leukocytes, but this decreases
to about 8% within 6 months of birth. The leukocytes are 80% neutrophils
and 15% macrophages, with the remaining 5% a mixture of other immune cells.
"Meanwhile, the epithelial-derived proportion of cells in breast milk rises
from as little as one percent at birth to up to 85% at six months
postpartum(Kordon & Smith 1998)."
Later on the same page (sigh), the authors contradict themselves a little by
writing that "macrophages account for 30 - 60% of the leukocytes in milk
during early lactation.... however numbers decrease markedly with progress
through lactation." They mention that individual differences are great, and
that neutrophils and eosinophils increase greatly in number (and therefore
decrease the macrophage concentration) when the risk of mammary gland
infection is high - eg early lactation, during mastitis or involution.
Milk macrophages are similar to those in other tissues, in that they can
engulf and digest bacteria - but they seem to have a host of other duties
too, even synthesising lysozyme and IgA antibodies and carrying
immunoglobulins. While it seems obvious why they should be so high around
birth and become more dilute as the milk volume increases, it's very
interesting to speculate on the relatively high and increasing proportion
of epithelial-derived cells in the milk. In fact, contrary to what I always
believed, the number of leukocytes decreases while the lactocytes from the
mother's ductal linings remains constant.
Lars Hanson's contribution in the "Textbook of Human Lactation" (Chapter 10)
is a much more readable and even more fascinating view of the "Role of
Breastfeeding in the Defense of the Infant".
"Does it mean to say that 80% of protein-cells are macrophages?"
Chapter 6 by Czank, Mitoulas and Hartman discusses protein and other
components in breast milk, but that's a whole different story.
So, Gonneke, it seems that your student's reference is not accurate with
regards to figures, and it's certainly not accurate with regards to
constancy of production of immune cells. It's an immensely complex subject.
But the important message there is correct - breast milk is a living fluid
that protects babies in many ways.
Best wishes
Jacquie Nutt
*********
Hi Gonneke,
The answer is inside your question: The student misread her reference. It
is not true that 80% of the 'contents' of milk is macrophages. 80% of the
cells in human milk are macrophages. Language is a tricky thing :)
Marcia McCoy
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