[Immunology] Re: What causes maternal antibodies to break down in a newborn?

Denise Gloor via immuno%40net.bio.net (by de_gloor from bluewin.ch)
Tue Dec 23 11:52:48 EST 2008


"Mike Clark" <mrc7 from cam.ac.uk> schrieb im Newsbeitrag 
news:b379120e50.mrc7offline from mrc7acorn1.path.cam.ac.uk...
> [Posted and mailed]
> In message <mailman.1085.1229101530.29717.immuno from net.bio.net>
>          "Alan Bradbury" <abradbury from taptonschool.co.uk> wrote:
>> I hope somebody can help with this.
>> A recent exam question for A level (16-18 year olds) in the UK had a
>> question asking why passive immunity in newborns is short lived.  The
>> answer expected was that a newborn will recognise the maternal
>> antibodies as foreign and have an immune response to them.  This just
>> didn't sound right to me. I have tried to do some internet research on
>> the question and it seems that the half-life of maternal antibodies is
>> about 30 - 45 days. But I have not been able to find out what causes
>> the removal of the maternal antibodies. I had assumed that it was just
>> due to the natural turnover and breakdown of the antibodies that the
>> infant might receive from its mother.
>> Am I right, or is the exam answer right?
>> Alan Bradbury (A level Biology teacher, UK).
> I would say that there is little evidence for the exam answer and that
> the observations are consistent with your answer.
> Human babies acquire IgG class antibodies from their mother by transport
> across the placenta in the third trimester of pregnancy. The
> transport receptor implicated for this is known variously as the
> Brambell receptor (after the academic who first postulated its
> existence) or the neonatel Fc receptor (FcRn). Thus young human infants
> acquire a polyclonal IgG reponse from their mothers.
> Once the baby is born the only immunoglobulin that the baby can acquire
> is in the colostrum and later the milk. Milk contains mainly IgM and IgA
> so this immunoglobulin will mainly pass through, and offer some passive
> immunity to, the digestive system. In some species such as ruminants and
> rodents there is little, if any, transplacental transport of IgG and
> most of the IgG immunoglobulin is transported across the gut by FcRn and
> is acquired from the colostrum. At present it has not been demonstrated
> that this is a major mechanism in humans for acquring IgG.
> Once the baby is born and stops acquring maternal IgG, the
> immunoglobulin will start to decline for two reasons. [1] Any
> immunoglobulin that encounters antigen will form an immune complex and
> be removed from the system via the complement and the Fc receptor
> mediated processes. [2] Immunoglobulin like any other protein is likely
> to be destroyed by natural catabolism and so will decline in
> concentration over time. Interestingly FcRn plays a major role for IgG
> in reducing the rate of catabolism and thus increasing the half-life of
> that particular class of antibodies.
> The examination answer suggests a third possibility. That the infants
> own anti-globulin response is responsible for removal of maternal IgG.
> This appears to be invoking an idea related to the Jerne anti-idiotype
> network hypothesis. That anti-idiotypes (acquired from the mother) will
> bind to the idiotypes of the infants new B-cells and stimulate them to
> produce antibody against the mothers antibody. Whilst this is
> theoretically a possibility I doubt whether it is the major cause of
> loss of maternal IgG, particularly since the observed decay is mainly
> consistent with [2] above.
> Cheers,
> Mike
> -- 
> M.R. Clark PhD,  Reader in Therapeutic and Molecular Immunology
> Cambridge University, Department of Pathology
> Tennis Court Road, Cambridge CB2 1QP
> Tel +44 (0)1223 333705    Web  http://www.path.cam.ac.uk/~mrc7/ 

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