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Studies on antibody functional affinity(35) and IgG subclass distribution(34)
in PPT suggest that, as in other autoimmune diseases, activation of the
complement system may have a role in pathogenesis.
The interaction of the complement system with thyroid antigen/autoantibody complexes has been quantitated by the use of two assay tech-niques, one based on the haemolysis of sheep erythrocytes in the presence of excess complement(37) and, the other, an ELISA for the quantitation of complement C3b immobilised as a result of classical complement pathway activation(38). It has been shown that not only is there activation of the complement system by thyroid directed autoantibodies(39), but that complement activation is related to the extent of the thyroiditis(40) and correlates with the severity of the thyroid dysfunction(41). (Fig. 5). In other forms of autoimmune thyroid disease (AITD), where complement activation has been suspected, it has been possible to measure elevated levels of the C5-C9 membrane attack complex (MAC) in the patients serum(42). However, despite an exhaustive search we have been unable to detect elevated MAC levels in serum samples from postpartum women who subsequently developed PPT(43). Figure 5:
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