The complement system is a crucial part of the innate immune system. In systemic autoimmune
disorders, its effects tend to be protective. On the contrary, in the autoimmune thyroid disorders (AIT)
the complement actively attacks thyrocytes, which express a number of complement components as well
as complement inhibitory molecules. According to the experimental studies different ways of
complement activation might occur in the thyroid tissue. All ensue via the classical pathway that is
started either by immune complexes containing complement activating autoantibodies against thyroid
autoantigens, or by direct binding of C4 to the molecule of thyroid peroxidase (TPO); or by direct
complement activation by reactive oxygen radicals. Thyrocytes are relatively resistant to the complement
attack. However, sublethaly injured thyrocytes release proinflammatory cytokines and reactive oxygen
radicals and thus promote the inflammatory process in the thyroid. So far, the clinical significance of the
complement in the thyroid has been studied only in postpartum thyroiditis. The exact role of complement
in the pathogenesis of AIT remains to be elucidated.
complement autoimmune thyroiditis, thyroid peroxidase, cytokines.