Breastfeeding and early infection in the aetiology of childhood leukaemia in Down syndrome.

Worldwide, acute leukaemia (AL) is the commonest type of childhood malignancy with an annual incidence of 30–50 cases per million children (Curado et al, 2007); Mexico City has one of the highest rates, at 58.4 cases per million (Mejia-Arangure et al, 2005a). Among the identified risk factors, individuals with Down syndrome (DS) below 20 years of age have 10- to 20-fold greater risk of developing AL compared with the general population (Robison, 1992; Ross et al, 2005). According to the Greaves' hypotheses, late contact with infectious agents may lead to an abnormal or aberrant immune response that favors the development of acute lymphoblastic leukaemia (ALL) (Greaves, 2005). Correspondingly, early infections have a protective effect in ALL by promoting the physiological maturation of the immune system, whereas late infections promote an excessive proliferation of the lymphocytes (McNally and Eden, 2004; Menegaux et al, 2004; Greaves, 2006; O'Connor and Boneva, 2007). Children with DS, due to their increased susceptibility to AL, provide a natural and valuable model for evaluating the interaction between the susceptibility for AL and the environment. In this study, we assessed whether breastfeeding and early infection were associated with the risk of developing AL in children with DS. Mother's milk is an important modulator of the immune response in infants because, while by providing the child with antibodies and immune cells, it can also be a source of transmission of infectious agents (MacArthur et al, 2008).