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Antibodies of class IgG to C1q are responsible for the formation of immune complexes. C1q is a glycoprotein consisting of 18 polypeptide chains and three unidentified subunits, in combination with two C1r and two C1s molecules forms the first complement component (C1). Activation of complement by the classical pathway is triggered when C1q is bound to immune complexes or to various other activating substances. After C1q binding, the conformational rearrangement of C1r and C1s occurs to proteolytic enzymes, which correspond to 4J for the continuation of activation along the classical pathway. In 1984, autoimmune antibodies to C1q in the serum of patients with systemic lupus erythematosus (SLE) were detected. These antibodies are found in large numbers in autoimmune diseases and kidney diseases. It was found, that the presence of autoimmune antibodies to C1q is directly associated with disease of immune complexes, especially with hypocomplexary urticarial vasculitis (the presence of antibodies to C1q is the main criterion) and diffuse proliferative lupus nephritis. Although it has not been proven that the availability of these antibodies to C1q is a diagnostic criterion for any of these diseases, determining the content of antibodies to C1q can be a valuable tool in the clinical examination of patients with SLE. Antibodies to C1q were observed in 15-60% of patients with SLE and in 95% of patients with lupus nephritis. A negative result of the determination of antibodies to C1q excludes the risk of kidney inflammation in the following months with a sensitivity of 95% (negative predictive value). In addition, the determination of autoimmune antibodies to C1q is important in monitoring patients, suffering from lupus. Successful treatment of active lupus nephritis with immunosuppressive drugs usually leads to a decrease in the content of these antibodies. The content of autoantibodies to C1q only slightly correlates with the content of antibodies to double-stranded DNA, but directly correlate with the content of C1q antigen.