Involvement of the complement system in immune thrombocytopenia: review of the literature

Immune thrombocytopenia (ITP) is a thrombocytopenic condition induced by autoimmune mechanisms and includes secondary ITP with underlying diseases such as connective tissue diseases (CTD). In recent years, it has been elucidated that the subsets of the ITP are associated with complement abnormalities but much remains unclear. To perform a literature review and identify the characteristics of complement abnormalities in ITP. PUBMED was used to collect the literature published up to June 2022 related to ITP and complement abnormalities. Primary and secondary ITP (CTD-related) were examined. Out of the collected articles, 17 were extracted. Eight articles were related to primary ITP (pITP) and 9 to CTD-related ITP. Analysis of the literature revealed that the ITP severity was inversely correlated with serum C3, C4 levels in both ITP subgroups. In pITP, a wide range of complement abnormalities was reported, including abnormalities of initial proteins, complement regulatory proteins, or the end products. In CTD-related ITP, reported complement abnormalities were limited to the initial proteins. Activation of the early complement system, mainly through activation of C3 and its precursor protein C4, was reported for both ITPs. On the other hand, more extensive complement activation has been reported in pITP.