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Complement is an intricate key element of the innate immune system. Being part of the first line of defense, it is mobilized rapidly to the expense of specificity. This lack of specificity necessitates a high degree of control which is revealed by its structure of a complex cascade of consecutive creation of complement components by highly regulated proteolytic steps.
Complement deficiencies predispose to an increased susceptibility to infection, in particular caused by Neisseria meningitidis, and to a minor extent by Streptococcus pneumonia and Haemophilus influenza. While Haemophilus influenzae is only a problem in early childhood and in immunocompromised subjects, the former two are important causes of life-threatening disease in youth and early adolescence or childhood and old age, respectively.