SOME children avoid developing asthma because they live with cats. Now a team
of immunologists has found out why, and it鈥檚 not the answer many researchers
expected.
The finding that cats can protect children against asthma was originally
thought to support the 鈥渉ygiene鈥� or 鈥渃leanliness鈥� hypothesis. According to this,
a lack of early exposure to bacteria or allergens keeps one branch of our immune
system, mediated by 鈥淭h1鈥� cells, from developing properly. As a result, the
immune system gets out of balance and the other branch, mediated by 鈥淭h2鈥� cells,
overreacts, causing the runny nose, sneezing and wheezing of allergic
reactions.
Supporters of the cleanliness hypothesis reasoned that exposure to cats might
be protecting against asthma by strengthening the Th1 system. To put the idea to
the test, Thomas Platts-Mills from the University of Virginia in Charlottesville
and his colleagues looked at the immune reactions of 226 American children to
cat allergens in their homes. Some were allergic to cats, and some weren鈥檛.
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They found that 1 in 5 of the children showed an immune response to cat
dandruff without having any allergic reaction. These children鈥檚 immune systems
produced an antibody called IgG4, rather than the IgE which causes allergies.
Both antibodies are part of the Th2 system. Platts-Mills believes IgG4 somehow
suppresses the harmful IgE response. 鈥淚 think this raises real questions about
the cleanliness hypothesis,鈥� he says.
The children with raised IgG4 clearly aren鈥檛 getting their protection from a
stimulated Th1 system, as the cleanliness hypothesis predicts. Instead the
protection is from a modified version of the Th2 response鈥攚ithout the risk
of asthma or the symptoms of other allergic reactions.
But Graham Rook, an immunologist at University College London, thinks the
work does support the cleanliness hypothesis after all. He says that many
researchers already reject the idea that immune problems are caused by an
imbalance between the Th1 and Th2 systems. Instead, he thinks that there鈥檚 a
problem with a system that regulates both responses鈥攁n idea which he says
is consistent with these new findings.
Either way, understanding why the Th2 system responds protectively could be
important for developing allergy therapies or 鈥渧accines鈥�, say the researchers.