Tolerance to malaria bad news

02 August 2009
Posted by Chris Smith.

A paper in the journal PLoS medicine this week has uncovered an intriguing insight into malaria. Malaria infects about 500 million people per year and the annual death toll is about 1 million and made up mostly of children in sub-Saharan Africa. The children most at risk are those aged between six months and three years. To find out why this group are so vulnerable Case Western Reserve University researcher Christopher King and his colleagues followed up 586 babies born in Kenya from birth to the age of three years.

A mosquitoThe team collected umbilical cord blood samples from the babies and a specimen of blood from each of the mothers. Tests showed that some of the mothers were malaria-infected at the time of delivery, indicating that the baby was most likely being exposed to products made by malaria parasites whilst in the womb. The team suspected that this might be causing the babies' immune systems to develop "tolerance" to malaria, a process by which the immune system learns when, we are first born, what it should befriend (and ignore) and what it should attack.

In keeping with this theory when the team mixed malaria antigens with white blood cells from some of the babies with infected mothers the cells reacted only very weakly to the malarial signal. White cells from babies who were not born to infected mothers, on the other hand, showed vigorous reactions and pumped out large amounts of inflammatory hormones. This suggests, say the team, that if a mother is infected with malaria when she is pregnant the baby's immune system is mislead into becoming tolerant to the parasite rather than attacking it.

This has serious implications for the development of a vaccine, say the team. Babies are protected from malaria for the first six months of their lives by antibodies from their mother. Once these are gone, say the team, the child becomes vulnerable. But if the baby's immune system thinks that malaria is a friend rather than a foe then re-programming this error could be essential for a vaccine to work.

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