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Mannose binding lectin (MBL) is a protein present in serum that is thought to play an important part in innate, non-specific immunity. It is, therefore, likely to be particularly important in young children before they have developed specific, exposure induced immunity. The gene (MBL2) which encodes for this protein is on chromosome 10. Three variant alleles are known and heterozygotes have 10–20% of normal serum MBL concentrations. There are also variant polymorphisms in the promoter region of the gene which affect serum concentrations of MBL. Danish researchers (Anders Koch and colleagues.JAMA2001;285:1316–21) have studied the MBL genotypes and susceptibility to acute respiratory infection of 252 children under the age of 2 years in Greenland. Thirteen of the children (5%) had MBL insufficient genotypes and the risk of acute respiratory tract infections, diagnosed by history and clinical examination, was doubled in these 13 compared with the other 239. The increase in risk was significant only in the 6–17 month age group. The prevalence of MBL insufficient genotypes in this population (81% Eskimo) is thought to be less than in many other populations. The innate immune system is particularly important in young children and mannose binding lectin is one component of it.