From the Guidelines
The C1 complement component plays a crucial role in the innate immune response, specifically in the classical complement pathway, where it recognizes and binds to antibody-antigen complexes, initiating the complement cascade and leading to inflammation, opsonization, and formation of the membrane attack complex. This function is essential for the elimination of pathogens and the maintenance of immune homeostasis. The C1 component is composed of three subcomponents: C1q, C1r, and C1s, which work together to activate the complement cascade 1. C1q is responsible for recognizing and binding to activators, while C1r and C1s are serine proteases that become activated upon C1q binding, leading to further progression of the complement cascade.
The importance of C1 in the immune response is highlighted by the fact that deficiencies in C1 can lead to increased susceptibility to infections and autoimmune disorders 1. The C1 component is also involved in the regulation of the immune response, and dysregulation of C1 can lead to inflammatory and autoimmune diseases.
Some key points to note about the C1 complement component include:
- C1 is the first component activated in the classical complement pathway
- C1 recognizes and binds to antibody-antigen complexes or other activating surfaces
- C1 is composed of three subcomponents: C1q, C1r, and C1s
- C1q is responsible for recognizing and binding to activators
- C1r and C1s are serine proteases that become activated upon C1q binding
- Deficiencies in C1 can lead to increased susceptibility to infections and autoimmune disorders.
Overall, the C1 complement component plays a critical role in the innate immune response, and its dysfunction can have significant consequences for immune homeostasis and disease susceptibility.
From the Research
Function of C1 in the Immune System
The C1 complex, composed of C1q, C1r, and C1s, plays a critical role in the classical complement activation pathway 2. The main functions of C1 can be summarized as follows:
- Initiator of the classical complement activation pathway 2
- Clearance of immune complexes and apoptotic bodies from the organism 3
- Removal of injured cells and debris 4
- Modulation of metabolic and regenerative processes 4
- Regulation of adaptive immunity 4
Activation and Control of C1
C1 activation can be induced by immune complexes or certain nonimmune substances 5. The activation of C1 is accompanied by limited proteolysis and conformational changes, and is controlled by the serum glycoprotein C1-inhibitor (C1-In) 5. C1-In blocks the enzymatic activities of activated C1 and efficiently disassembles C1, thereby releasing two inactive C1rC1s(C1-In)2 complexes per C1 molecule 5.
Clinical Significance of C1 Deficiency
C1 deficiency results from an absence or lowering of the level of one or more of the proteins C1q, C1r, and C1s, and is associated with an inability to deal effectively with immune complexes, resulting in immune-complex-related diseases and a great susceptibility to recurrent bacterial infections 6. Deficiency of C1s is also associated with early-onset systemic lupus erythematosus and increased susceptibility to bacterial infections 2.