WOMEN have a weaker immune response to allergens with each successive pregnancy, which might explain why children born later tend to suffer less from eczema, hay fever and asthma than their older siblings.
Wilfried Karmaus at Michigan State University in East Lansing and his colleagues had already showed that at birth, firstborn children have the highest levels of IgE, the key immune system protein responsible for allergies.
In a survey of 820 mothers and their newborn babies from the Isle of Wight in the UK, the team has now found that maternal levels of IgE are also highest with the firstborn and drop with each pregnancy (Clinical and Experimental Allergy vol 34 p 853). That suggests that the mother鈥檚 immune response is somehow influencing the baby鈥檚 immune system, say the researchers. This may affect the child鈥檚 sensitivity to allergens later.
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The finding hints at an alternative to the 鈥渉ygiene鈥� theory, the most popular explanation for why children born later have fewer allergies. According to this theory, more older siblings means more dirt and infections, which primes the immune system so that the child is less likely to develop harmful hypersensitivities.