Scientists believe they have uncovered why people with a gene for a blood disorder are immune to malaria.
The red blood cells are scythe shaped in sickle cell anaemia
It is known that people with a single gene for sickle cell anaemia, but not the full-blown condition, are somewhat resistant to the malaria parasite.
Some say the distorted red blood cells caused by the gene are broken down quicker than normal by the body so malaria has no home in which to thrive.
Now a Wellcome Trust team suggests the immune system also plays a big role.
By studying more than 1,000 children living on the coast of Kenya, where malaria is rife, they found many with the sickle cell trait developed increasing protection against malaria as they aged.
Between the ages of two and 10, immunity to the disease rose rapidly.
The protection against malaria was around 20% in the first two years of life compared with over 50% by the age of 10, the researchers report in the journal PLoS Medicine.
However, protection is not 100%, so people with the trait still need to be aware of the risks of malaria, they stressed.
Sickle cell anaemia
Inherited disorder affecting red blood cells
Named after scythe-type shape of red blood cells
Those with two faulty genes have full-blown disease
Those with only one copy have "sickle cell trait"
More common in African and Afro-Caribbean people and malarious areas
Children with sickle cell trait do not usually display symptoms of full-blown sickle cell anaemia, such as breathlessness and bone and joint pain
Lead researcher Dr Tom Williams said: "It has been known for some time that sickle cell trait offers this protection, but the accelerated level of immunity in the first years of life has not been revealed before."
He said there were several possible reasons why this happened, but that further research was needed to know for sure.
One explanation might be that although the bulk of blood cells carrying malaria are destroyed quickly, a few may escape destruction - but not enough to cause malaria symptoms.
This would allow the immune system time to gradually build up an effective defence against malaria, he suggested.
He cautioned that his team had studied only mild malaria, which causes fever and does not kill.
"We do not know if our findings can be applied to severe forms of malaria, which can lead to death," he said.
Dr Colin Sutherland, of the Health Protection Agency's Malaria Reference Laboratory at the London School of Hygiene & Tropical Medicine, said: "This work has provided us with tantalising evidence that mild sickle cell trait does more than just impede the malaria parasite's growth in the abnormal red cell, it actually enhances the child's acquisition of specific anti-malaria immunity.
'"If these results are confirmed by further studies, then children with the mild form of sickle cell anaemia will have provided a new model for understanding the natural process of becoming immune to malaria."
Sickle cell trait occurs when someone inherits a normal gene from one parent and a sickle cell gene from the other parent.
Children with sickle cell trait do not usually display the symptoms of those with full-blown sickle cell anaemia, such as breathlessness and bone and joint pain.
In sickle cell disease, the major protein in red blood cells that carries oxygen around the body, haemoglobin, is different from normal.
Although it can still carry oxygen, the red blood cells tend to distort into a sickle shape, instead of a doughnut shape, and can disrupt smooth blood flow, which leads to complications.