Michaela found the following
extract in Roit Mastoff and Male, (1996), Immunology 4th Ed., Times Mirror
International Publishers Spain,
Commentary is between Michaela and Alan Beat.
All I had to do was consult the book.
IgA is detected in seramucous secretions i.e. saliva, milk, tracheobronchial and genitourinary secretions. IgA becomes focused atmucosal surfaces where it prevents reinfection.
(Antibodies or Ab are the old fashioned name for immunoglobulins Ig),
of which there are 5 classes. IgG is the major Ig making up
about 70-75% of the total immunoglobulin pool. IgM accounts for about 10% and
is the predominant 'early' antibody seen in response to infectious
has the ability to be both specific and general in its response to infection. They have epitopes which recognise and bind specific antigen e.g. FMDV and the mechanism for testing is as follows:
Immunonassay or elisa FMDV (Ag) is incubated on a plastic plate, and small quantities are absorbed into the plastic. Free Ag is washed away. Test Ab is added which is labelled and will bind to the Ag. Again washing takes place and the unbound parts are washed away. In elisa, a chromogen is added, which produces a colouredeffect making detection of Ab that much easier optically.
I have done these tests myself while at university and the outcome is
dependent on the skill of the individual carrying them out!
You can see that if the sample contains no specific FMDV antibodies it
cannot bind to the prepared plate and it will all be washed away and the
result is negative.
Our further questions: Can you say how significant IgA is? Does it "neutralise" any low-level live virus residues in the throat? Do the Elisa tests check for just one class of Ig?
IgA is the antibody that is predominantly found in mucous type secretions as stated and yes it will tend to 'bind'/'lock/ neutralise virus in the nose, mouth and throat.
IgA will be present later than IgG or IgM,
both of which circulate in the bloodstream. So when animal is infected
the background levels increase. The rapidity of the response is dependent
upon prior exposure (to any infections). This is the reason why mature
healthy animals and humans are less likely to become ill than the young the
stressed and the old. The young are immunologically defined as 'immature', the
old as incompetent, the stressed as 'depressed'. When exposed to any
acute infection, up shoot IgM Ab first. This is how, apart from taking a
temp, checking to see whether the
animal is listless, depressed off its food and so on, that it is acutely ill!
A blood sample is taken and yes it is
possible to determine the various immunoglobulins by individual particular characteristics dependent upon electrophoresis ( Ab/Ig are proteins, and proteins display specific characteristics when probed by an electrical current. They tend to move a characteristic distance from the stimulus!).
This information has been confirmed by Dr Simon Barteling (Elm Farm meeting),
and also Dr Ruth Watkins, virologist (Wolverhampton and Sennybridge meetings).
If you were looking at a graph for a rise in Ab following an acute infection, first up goes IgM, followed by IgG, but both have binding sites that would 'recognise' specific infection and as I explained in the last email, when subjected to elisa and the specific Ab (FMD) or Ag (FMDV) that is being tested for is labelled, it is then possible to determine definitively that the animal either has the virus or the Ab, but we are talking 2 different elisa tests. One for virus, one for Ab. method is the same, prep different.
Our comment: Thanks for that explanation, Michaela, it takes a bit of understanding but the bottom line is this - antibody present in the saliva neutralises live virus in a "carriers" throat and prevents infection being
passed on to other animals. We presume this is the reason why "carrier" animals represent no threat in practical terms to other livestock, and why all laboratory attempts to demonstrate that cross-infection can occur have