Immunoglobulin G in the context of Complement component 1q

The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM.

Following activation, a series of proteins are recruited to generate C3 convertase (C4b2b, historically referred C4b2a), which cleaves the C3 protein. The C3b component of the cleaved C3 binds to C3 convertase (C4b2b) to generate C5 convertase (C4b2b3b), which cleaves the C5 protein. The cleaved products attract phagocytes to the site of infection and tags target cells for elimination by phagocytosis. In addition, the C5 convertase initiates the terminal phase of the complement system, leading to the assembly of the membrane attack complex (MAC). The membrane attack complex creates a pore on the target cell's membrane, inducing cell lysis and death.

Childhood-onset systemic lupus erythematosus (i.e., cSLE), also termed juvenile-onset systemic lupus erythematosus, juvenile systemic lupus erythematosus, and pediatric systemic lupus erythematosus, is a form of the chronic inflammatory and autoimmune disease, systemic lupus erythematosus (SLE), that develops in individuals up to 18 years old. Early-onset systemic lupus erythematosus is often used to designate a subset of cSLE patients who are up to 5 years old. Children with early-onset SLE tend to have a more severe form of cSLE than children who develop cSLE after 5 years of age.

cSLE does not include neonatal lupus erythematosus (nSLE). nSLE is a SLE-like disease that is present in infants at birth. It is caused by certain antinuclear antibodies, e.g., the immunoglobulin G types of the anti-SSA/Ro autoantibodies (e.g., anti-Ro/SS-A and anti-La/SS-B) and anti-nRNP (also termed anti-U1RNP). These antibodies form in the mother and pass from her circulation through the placenta to the fetus where they cause an often severe form of SLE that is evident in the fetus and newborn child. Most of the disorders in the infants disappear within months as these antibodies are naturally cleared from the infant. However, one disorder occurring in nSLE, congenital heart block, usually does not reverse and is potentially lethal. Fetuses and neonates with this heart block are implanted with an artificial cardiac pacemaker. However, recent studies have shown that hydroxychloroquine given to the mother in her 6th and 10th gestational weeks or intravenous immunoglobulin therapy given to the mother in her 14 and 18 gestational weeks reduces the incidence of developing this heart block (Intravenous immunoglobulins given to the mother suppress her production of antibodies including those that cause nSLE.).