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Autoimmunity

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Autoimmunity - A Comprehensive Overview

1. Definition

Autoimmunity is the failure of the immune system to distinguish self from non-self, leading to immune responses directed against the body's own tissues. It arises from a breakdown of immunological self-tolerance - the normally acquired state of unresponsiveness to self antigens. When these mechanisms fail, self-reactive T and B lymphocytes generate cell-mediated or humoral immune attacks against host tissue, causing autoimmune disease.
  • Robbins & Kumar Basic Pathology, p.176
  • Janeway's Immunobiology 10e, p.716

2. Self-Tolerance: The Foundation

Normal immunity prevents autoimmunity via two layers of tolerance:

Central Tolerance

Occurs during lymphocyte development in the thymus (T cells) and bone marrow (B cells):
  • Developing lymphocytes that recognize self antigens too strongly undergo clonal deletion (apoptosis)
  • The AIRE (Autoimmune Regulator) gene in medullary thymic epithelial cells drives expression of peripheral tissue antigens, enabling deletion of organ-specific self-reactive T cells. Mutations in AIRE cause Autoimmune Polyendocrine Syndrome (APS)
  • In B cells, receptor editing redirects strongly self-reactive BCRs

Peripheral Tolerance

Catches self-reactive cells that escape the thymus/bone marrow:
  • Anergy - T cells encountering antigen without costimulation become functionally unresponsive
  • Regulatory T cells (Tregs) - FOXP3+ cells suppress effector lymphocytes via IL-10, TGF-β, and CTLA-4. Loss of FOXP3 causes the devastating IPEX syndrome (immune dysregulation, polyendocrinopathy, enteropathy, X-linked)
  • Activation-induced cell death (AICD) via Fas/FasL - mutations cause Autoimmune Lymphoproliferative Syndrome (ALPS)
  • Immune privilege - certain anatomical sites (eye, testis, CNS) sequester antigens from the immune system
  • Cellular and Molecular Immunology (Abbas), p.1019
  • Robbins & Kumar Basic Pathology, p.176
Gene DefectMouse PhenotypeHuman Disease
AIREEndocrine organ destructionAPS
FOXP3Multi-organ lymphocytic infiltratesIPEX
FAS/FASLAnti-DNA antibodies, lymphoproliferationALPS
CTLA4Lethal lymphoproliferation by 3-4 wksSystemic inflammatory disease
C4SLE-likeSLE
IL2/IL2RIBD, anti-erythrocyte autoantibodiesMulti-system autoimmunity
- Cellular and Molecular Immunology, Table 15.5, p.1019

3. Pathogenesis of Autoimmunity

Autoimmune disease is the product of susceptibility genes interacting with environmental triggers:
Pathogenesis of autoimmunity - susceptibility genes and environmental triggers converge to activate self-reactive lymphocytes causing tissue injury
From Robbins & Kumar Basic Pathology - Susceptibility genes cause failure of self-tolerance, while environmental triggers (infection, tissue injury, inflammation) activate self-reactive lymphocytes, resulting in tissue damage.

3a. Genetic Factors

  • Autoimmune diseases cluster in families; monozygotic twins have higher concordance than dizygotic
  • Strong associations with HLA alleles (particularly HLA-DR and HLA-DQ):
    • HLA-B27 in ankylosing spondylitis: relative risk ~100
    • HLA-DR4 in rheumatoid arthritis: relative risk ~3-4
    • HLA associations with SLE, T1D, MS are weaker
  • GWAS has identified many non-HLA polymorphisms - many in non-coding regions affecting immune activation and regulation, and shared across multiple diseases
  • Robbins & Kumar Basic Pathology, p.177

3b. Environmental Triggers

Infections are the most important environmental trigger, operating via two main mechanisms:
Role of infections in autoimmunity - costimulator induction on APCs (pathway A) and molecular mimicry (pathway B)
A. Upregulation of costimulators: Infection activates APCs to express costimulatory molecules (B7/CD28 axis) and secrete T-cell-activating cytokines. When these APCs present self antigens in this inflammatory context, self-reactive T cells are activated rather than anergised. Toll-like receptor (TLR) engagement by microbes on dendritic cells also drives type I interferon production (particularly relevant in SLE).
B. Molecular mimicry: Microbial antigens share cross-reactive epitopes with self antigens. The classic example is rheumatic fever - anti-streptococcal antibodies cross-react with myocardial proteins, causing myocarditis.
Other environmental factors:
  • UV radiation causes cell death and exposure of nuclear antigens - explains SLE flares with sun exposure
  • Smoking (risk factor for RA - possibly through chemical modification of self antigens)
  • Release of sequestered antigens (ocular, testicular) following trauma
  • Epitope spreading - as autoimmune tissue injury proceeds, new self epitopes are exposed, broadening the response and driving chronicity
Gender bias: Most autoimmune diseases predominate in women (SLE 9:1 F:M). Mechanisms not fully established but likely involve hormonal effects on immune regulation.
  • Robbins & Kumar Basic Pathology, p.177-178
  • Cellular and Molecular Immunology, p.1020-1021

4. Key Autoimmune Diseases

Systemic Lupus Erythematosus (SLE)

A multi-system autoimmune disease defined by production of antinuclear antibodies (ANAs), particularly anti-dsDNA and anti-Sm. Prevalence up to 400/100,000 in some populations; F:M ratio 9:1 in reproductive years.
Autoantibodies and their significance:
AntibodyPrevalenceClinical Significance
ANA (generic)~95%Screening test
Anti-dsDNA~70%Highly specific; correlates with disease activity and nephritis
Anti-Sm~25%Virtually diagnostic of SLE
Anti-Ro (SSA)~30%Neonatal lupus, SCLE
Anti-La (SSB)~20%Neonatal lupus
Antiphospholipid~50%Thrombosis, recurrent miscarriage
Anti-histoneVariesDrug-induced SLE
Pathogenesis: Failure of apoptotic cell clearance leads to persistence of nuclear antigens → nuclear antigen-antibody complexes engage TLRs on B cells and dendritic cells → amplified autoantibody production + type I interferon production → cycle of antigen release and immune activation.
Tissue injury mechanisms:
  • Type III hypersensitivity (immune complex deposition) - glomerulonephritis, vasculitis
  • Type II hypersensitivity (antibody-mediated) - hemolytic anemia, thrombocytopenia, leukopenia
  • Antiphospholipid antibodies - arterial and venous thrombosis, miscarriage
  • Robbins & Kumar Basic Pathology, p.178-183

Rheumatoid Arthritis (RA)

Chronic inflammatory arthritis driven by immune complex formation. Rheumatoid factors are IgM antibodies against the Fc portion of IgG; they form immune complexes that activate complement, causing chronic synovial inflammation. Anti-CCP antibodies are more specific. HLA-DR4 is the key genetic association.

Multiple Sclerosis (MS)

Cell-mediated autoimmune attack on CNS myelin by autoreactive T cells (particularly Th1 and Th17 cells). The primary target antigens include myelin basic protein (MBP) and myelin oligodendrocyte glycoprotein (MOG). Results in demyelination, axonal damage, and cumulative neurological disability.

Type 1 Diabetes Mellitus (T1DM)

Cell-mediated destruction of pancreatic islet β cells by autoreactive CD4+ T cells (Tfh cells) and cytotoxic CD8+ T cells, with macrophage-mediated lytic enzyme production. Autoantibodies (anti-GAD65, anti-IA-2, anti-insulin) are present but do not significantly cause disease - they serve as markers. HLA-DR3 and DR4 are the main risk alleles.

Other Major Autoimmune Diseases

DiseaseDominant MechanismTargetKey Autoantibody
Hashimoto's thyroiditisT cell-mediated + antibodyThyroid peroxidaseAnti-TPO, anti-Tg
Graves' diseaseStimulatory antibody (Type II)TSH receptorAnti-TSHR
Myasthenia gravisBlocking antibody (Type II)ACh receptorAnti-AChR
Goodpasture syndromeAnti-GBM antibody (Type II)Glomerular basement membraneAnti-GBM/collagen IV
Pemphigus vulgarisAntibody against cell junctionDesmoglein 3Anti-desmoglein
IBD (Crohn's/UC)T cell-mediatedGut mucosaVarious
Sjögren's syndromeT cell + B cellExocrine glandsAnti-Ro/La
  • Katzung's Basic & Clinical Pharmacology 16e, p.1527
  • Cellular and Molecular Immunology, p.1276

5. Mechanisms of Tissue Injury

Autoimmune damage maps to the Gell-Coombs hypersensitivity classification:
TypeMechanismExamples
Type IIgE-mediated (not typical in autoimmunity)-
Type IIAntibody against cell/tissue antigenHemolytic anemia in SLE, Graves', Myasthenia, Goodpasture
Type IIIImmune complex depositionLupus nephritis, vasculitis, serum sickness
Type IVT cell-mediated (DTH)MS, T1DM, Crohn's disease, contact dermatitis
Th17 cells (producing IL-17) play a major pathogenic role in psoriasis, IBD, RA, MS, and SLE.

6. Diagnostics

Autoantibody Testing

  • ANA (antinuclear antibody): the primary screening test. Nearly all SLE patients are ANA-positive, but a positive ANA is not specific (healthy individuals, other diseases). Pattern of immunofluorescence (homogeneous, speckled, nucleolar, rim) gives clues.
  • Anti-dsDNA + anti-Sm: virtually diagnostic of SLE
  • Rheumatoid factor (RF) + anti-CCP: RA
  • Anti-TPO: Hashimoto's thyroiditis
  • ANCA (anti-neutrophil cytoplasmic antibody): vasculitides - c-ANCA (anti-PR3) in granulomatosis with polyangiitis; p-ANCA (anti-MPO) in microscopic polyangiitis
  • Anti-AChR: Myasthenia gravis
  • Anti-GBM: Goodpasture syndrome
Complement levels (C3, C4, CH50) are consumed during active immune complex disease (e.g., lupus nephritis flares), making them useful disease activity markers.

7. Treatment Principles

Current therapy targets immune activation at multiple levels (Cellular and Molecular Immunology, p.1296; Katzung, p.1527):

Non-specific Immunosuppression

  • Corticosteroids - broad anti-inflammatory; first-line for most acute disease
  • Methotrexate - folate antagonist; antiproliferative; cornerstone of RA therapy
  • Azathioprine / mycophenolate mofetil - purine synthesis inhibitors; reduce lymphocyte proliferation
  • Cyclophosphamide - alkylating agent; used in severe lupus nephritis, vasculitis

Biologics (Targeted)

  • Anti-TNF agents (infliximab, etanercept, adalimumab) - RA, IBD, psoriasis
  • Anti-IL-6 (tocilizumab) - RA, cytokine storms
  • Anti-IL-17 (secukinumab, ixekizumab) - psoriasis, ankylosing spondylitis
  • Anti-IL-12/23, anti-IL-23 (ustekinumab, risankizumab) - IBD, psoriasis
  • Anti-CD20 (rituximab) - B cell depletion; SLE, RA, vasculitis
  • Anti-CTLA-4-Ig (abatacept) - blocks T cell costimulation (B7-CD28); RA
  • Belimumab (anti-BLyS/BAFF) - B cell survival factor; approved for SLE
  • Natalizumab - anti-α4 integrin; blocks lymphocyte trafficking to CNS in MS

Future Directions

The ultimate goal is antigen-specific tolerance induction - restoring tolerance specifically to the self antigen driving disease without global immunosuppression. Approaches under investigation include regulatory T cell therapy, tolerogenic dendritic cells, and CAR-Treg strategies.

8. Key Concepts Summary

ConceptKey Point
Self-toleranceMaintained by central deletion (thymus/BM) + peripheral mechanisms (anergy, Tregs, AICD)
Genetic basisHLA locus (especially DR/DQ) + many non-HLA GWAS variants; multigenic, not monogenic
Environmental triggerInfections (costimulator upregulation, molecular mimicry), UV, smoking, sequestered antigens
Epitope spreadingAutoimmune tissue damage releases new antigens, broadening and perpetuating the response
Gender bias~80% of autoimmune disease patients are women; hormonal mechanisms likely
Organ-specific vs. systemicOrgan-specific (Hashimoto's, T1DM, MS) vs. systemic (SLE, RA, Sjögren's)
TherapyNon-specific immunosuppression + targeted biologics; future goal = antigen-specific tolerance
Sources: Robbins & Kumar Basic Pathology (Elsevier, 2023) | Cellular and Molecular Immunology, Abbas et al. (Elsevier, 2022) | Janeway's Immunobiology 10e | Katzung's Basic and Clinical Pharmacology 16e | Firestein & Kelley's Textbook of Rheumatology | Goldman-Cecil Medicine
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