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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