3.3 Dermatomyositis

Inflammatory myopathy is a group of acquired diseases characterized by inflammatory cell infiltration of skeletal muscle and muscle fiber necrosis, usually manifested as moderate to severe muscle weakness and infection, mainly including polymyositis, dermatomyositis and inclusion bodies. Dermatomyositis (DM) is a type of microvascular disease regulated by complement, which mainly affects the skin and muscles. The activation and deposition of complement can cause endocardial capillary rupture, resulting in muscle ischemia. The cleaved vascular endothelium serves as the main target antigen and can trigger antibody production. The antibody directly binds to endothelial cells, thereby activating C3. Activated C3 further produces C3b and C4b fragments, which ultimately leads to the formation of MAC. Complement deposition will continue to cause endothelial cell enlargement and vacuolization, eventually leading to ischemia and muscle fiber destruction.

3.4 Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) mainly includes cryoglobulinemia vasculitis and allergic purpura nephritis. The former is due to excessive complement consumption, patients usually have hypocomplementemia, manifested by low concentrations of C4 and Clq in the serum, C3 and Clq can be detected in the tissue. The latter usually has IgA deposition, and at the same time on the glomerular C3 deposition similar to IgA nephritis occurs. It is currently believed that IgA deposition in allergic purpura nephritis and IgA nephritis is related to the deposition of complement factors, mainly C3, P factor and MAC.

3.5 Anti-glomerular basement membrane disease

Anti-glomerular basement membrane disease refers to a group of autoimmune diseases caused by the deposition of circulating anti-glomerular basement membrane antibodies in the organs. The disease is caused by IgG autoantibodies attacking the glomerular basement membrane (GBM), at the same time, because the alveolar basement membrane and GBM have a certain homology in structure, it can involve the lungs and lead to pulmonary hemorrhage-nephritis syndrome. The disease is one of the few autoimmune diseases for which antigens have been identified so far, and pathogenic antigens can cause the production of anti-IgG autoantibodies. After the antigen and antibody are combined, they can produce autoimmune reactions, such as strong complement activation, leukocyte infiltration and proteinuria, which will eventually lead to the formation of crescents and scars, leading to the decline of kidney function.

3.6 Antiphospholipid syndrome

Antiphospholipid syndrome (APS) is a group of clinical signs of thrombosis, habitual abortion, thrombocytopenia, and neuropsychiatric system damage caused by antiphospholipid antibodies (APL antibodies). APL antibody can react with a variety of antigenic substances containing phospholipid structure include lupus anticoagulant, anticardiolipin antibodies, antiphosphatidic acid antibodies and antiphosphatidylserine antibodies. Complement activation plays a key role in miscarriage and fetal growth retardation in APS patients. The levels of C3 and C4 in the serum of early APS patients are low, but the levels of C3a and C4a are higher than normal people, indicating that the hypocomplementemia in APS patients is caused by complement activation rather than complement defects.

3.7 Systemic sclerosis

Systemic sclerosis (SSc) is a type of connective tissue disease characterized by thickening of the skin and visceral fibrosis, which can affect the peripheral parts and the face (limitation), or the proximal extremities and trunk (diffuse). Studies have shown that there is no complement deposition in SSc patients. However, there is immune complex deposition in endothelial cells, accompanied by abnormal complement activation. The study found that the plasma C3d levels, C3d / C3 and Ba / B factor ratios of patients with diffuse SSc were higher than normal people, and C3d, C3d / C3, C4d, C4d / C4 of patients with restrictive SSc were higher than normal people, which suggests that SSc may be via the classical pathway activation, its level can be used as an indicator of clinical detection of disease.

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