Complement System and Immune Regulation

Canonical source: hsa04610

Pathway Overview

The complement cascade is part of innate immunity, mediating opsonization, inflammation, and lysis. C3 activation triggers the alternative pathway; C4 participates in classical and lectin pathways. HLA-DRB1 is in the MHC region and associates with complement gene variation; C4A copy number and HLA variants are strongly linked to autoimmune disease. The complement system is implicated in multiple sclerosis (C4-mediated synaptic pruning), rheumatoid arthritis (immune complex deposition), and schizophrenia (complement in synaptic elimination). UV exposure and vitamin D may modulate autoimmune risk via immunomodulation. HLA-DRB1 variants influence susceptibility to rheumatic and neuroinflammatory disease.

Environmental Triggers

ExposureTrigger type
uv-radiationUV and vitamin D modulate immune function; latitude gradient in MS
tobaccoSmoking increases rheumatoid arthritis risk; may trigger citrullination and immune activation
endocrine-disruptorsHormone disruption may affect autoimmune disease onset

Genetic Modulation Points

Key genes

  • hla-drb1— MHC class II; antigen presentation; shared epitope in rheumatoid arthritis; strongest genetic risk factor for MS
  • il33— Alarmin; bridges innate and adaptive immunity; relevant to inflammatory and allergic disease

Regulatory checkpoints

  • C3 convertaseTherapeutic target
  • Antigen presentation— hla-drb1

Tissue Specificity

central nervous system— Complement in synaptic pruning; MS pathogenesis
synovium— Rheumatoid arthritis; immune complex deposition

Disease Relevance

Linked diseases

  • multiple-sclerosis— HLA-DRB1 and C4; complement-mediated synaptic elimination; vitamin D interaction
  • rheumatoid-arthritis— HLA-DRB1 shared epitope; smoking-citrullination axis; complement in inflammation

Linked exposures

  • uv-radiation— Vitamin D from UV may protect against MS; immunomodulation
  • tobacco— Smoking increases RA risk; interacts with HLA-DRB1

Pathway Diagram

Pathway diagram placeholder. A visual representation of this pathway will be integrated when available.

Evidence Nodes

Evidence for this pathway is derived from:

  • 3 environmental trigger(s)
  • 2 key gene(s)
  • 2 linked disease(s)
  • 2 linked exposure(s)

References

  1. 1.Ramagopalan SV, et al. (2009). HLA-DRB1, vitamin D, and multiple sclerosis. PLoS Genetics. doi:10.1371/journal.pgen.1000369
  2. 2.Källberg H, et al. (2011). Smoking and HLA-DRB1 in rheumatoid arthritis. Annals of the Rheumatic Diseases. doi:10.1136/ard.2010.129924
  3. 3.Lucas RM, et al. (2011). UV exposure and multiple sclerosis. Neuroepidemiology. doi:10.1159/000328158
  4. 4.Ricklin D, et al. (2016). Complement in autoimmune disease. Nature Immunology. doi:10.1038/ni.3481