Leukemia is more common in children with Down syndrome, being seen anywhere from 10 to 30 times more often than in the general population of children. The vast majority of cases occur in the first 5 years of life.
In the first 3 years of life, nonlymphoid leukemia is the most common form of leukemia in children with Down syndrome; after age 3, approximately 80% have acute lymphocytic leukemia and 20% have nonlymphocytic leukemia. It is worth noting here that children with Down syndrome who develop AML seem to respond to chemotherapy better than do children without Down syndrome; with ALL, the response rate appears to be about the same.
There are also functional defects of white blood cells in Down syndrome. White blood cells in people with Down syndrome have a decreased response to infection, and a decreased killing ability of microorganisms. This may be one reason for the decreased immunity to infection seen in children with Down syndrome.