The complement system is a major, multifunctional part of innate immunity and serves as a bridge between the innate and adaptive immune systems. It consists of more than 30 distinct proteins that interact with one another in a specific sequence. There are three pathways of complement activation: the classical, the lectin, and the alternative pathways. The three pathways are initiated by distinct mechanisms, but they all generate the same core set of effector molecules. Inherited complete deficiencies in complement components are generally very rare and predispose to infections and autoimmune disease. One of the better described associations is between deficiencies in early classical pathway components and the development of systemic lupus erythematosus. The goal of this review will be to discuss the associations between and the causal mechanisms of complement deficiencies and systemic lupus erythematosus.

Juvenile idiopathic arthritis (JIA) is the most common rheumatic disease in childhood. Enthesitis-related arthritis (ERA) is one of the seven JIA subtypes classified by the International League of Associations for Rheumatology (ILAR). Due to inclusion and exclusion criteria, a pitfall of the ERA category is that it does not include all subsets of juvenile spondyloarthropathy, with many children ending up in the undifferentiated category. The ERA nomenclature also does not have a method for distinguishing between axial and peripheral disease, two phenotypes which vary in presentation and treatment requirements. This distinction is very important given the overall poor prognosis seen in ERA patients, specifically in those with axial involvement. Since axial involvement is more common and presents earlier than previously thought in ERA, the pediatric rheumatology community should develop more accurate and sensitive classification criteria based on disease course to assist in improving timely diagnosis and appropriate management.