Here's a study that shows what a useful tool IgA is in breastmilk for
detection of bacteria and viruses.  Probably the why of this component's use
in various test kits such as ELISE or Western Blot.  While this study
represents the wonders of breastmilk, notice that the end result is to make a
vaccine for use in young infants.  I assume those young infants won't be the
breastfed ones.

http://www.asmusa.org/pcsrc/gm2001/34752.htm
"Human breast milk contains immune molecules called IgA antibodies, which
allow us to detect components of dangerous microorganisms to humans (pathogen
bacteria and virus). By using these antibodies we have detected virulent
proteins that were injected by the bacteria into the cells after these
microorganisms attach to the intestinal mucosa. The bacteria called
Enteropathogenic Escherichia coli (EPEC) is a bacterial pathogen that adheres
to human intestinal cells, resulting in persistent diarrhea in infants and
young children. We have shown that IgA antibodies isolated from breast milk
from Mexican women block the adherence of the bacteria to human cells and
reacted with several virulent proteins that have been transferred by the
bacteria into the intestinal cells.Our results indicate that IgA antibodies
from human milk may be one of the main factors by which human milk protect
newborns against enteric bacterial pathogens. The information obtained in the
present study is useful in the development of a future vaccine to protect
young infant against these pathogens."

This study also shows that IgA inhibited the adherence of E-coli strains to
cells.  But, of course, this knowledge will help us make a wonderful vaccine.
I would think that one would want to put more energy/funds into promoting
breastfeeding.  But I guess that is silly of me to think that this would be
done.  Instead we will make a vaccine and some company will make alot of
money.  Valerie W. McClain, IBCLC

More quotes from this study...
"Human milk has been found to be an important protective factor against
infectious diseases. Several studies have shown that colostrum and human milk
protect newborn children against gastrointestinal infections. We have
previously shown that human secretory IgA from breast milk inhibited the
adherence of EPEC strains to human cells.
The IgA antibodies from breast milk can be used as a tool to detect virulent
proteins from pathogenic microorganisms since the hard work (selection of the
virulent proteins and production of the specific antibodies) has already done
by the immune system of the mother. The aim of the present study was to
search for virulence antigens transferred from EPEC to human cells using IgA
antibodies. Detection of the EPEC-infected cells by IgA antibodies showed
several virulent proteins that were transferred into the human cells.
Preliminary experiments suggested the identity of three of these proteins; 1)
Tir (Translocated intimin receptor) which is injected by the bacteria into
the intestinal cell and functions as a receptor for its binding partner,
intimin, resulting in a pedestal-like structure upon which the bacteria
resides; 2) EspB a secreted protein that participates in the production of
lesions to the intestinal cell; 3) EspC, a secreted protein that has an
unclear role in the pathogenesis of EPEC-associated intestinal damage.
This reaction to EPEC is due to exposure of woman donors to EPEC strains that
have induced an antibody response in their milk. This explanation is
supported by a preliminary study that showed that milk samples from women
living in poor sanitary conditions have a stronger IgA response to EPEC than
milk samples from women living in good sanitary conditions. Our results
suggest that the virulent proteins injected by EPEC into the intestinal cells
stimulate an immune response and may elicit protective immunity against EPEC
disease."

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