Identical twins share the same HLA alleles, yet concordance for most autoimmune diseases is only 30–50%. What does this most directly imply about autoimmune disease causation?
AHLA alleles are not actually the major genetic risk factor; other loci must account for most of the variance
BEnvironmental triggers are required to convert genetic susceptibility into active disease — HLA is necessary but not sufficient
CThe immune system of one twin must have compensatory mechanisms absent in the other
DAutoimmune disease is primarily epigenetic, so identical DNA does not predict identical outcomes
If autoimmune disease were purely determined by HLA genotype, identical twins would have near-100% concordance. The 30–50% figure shows that HLA creates susceptibility but cannot cause disease alone. Environmental triggers — infection, molecular mimicry, microbiome changes — must act on genetically susceptible individuals to initiate loss of tolerance. This is why genetically identical people raised in different environments differ in disease incidence.
Question 2 Multiple Choice
A patient develops anti-TSH receptor antibodies that continuously stimulate thyroid hormone production, causing hyperthyroidism (Graves' disease). Which autoantibody mechanism does this illustrate?
AImmune complex deposition: antibodies bind the receptor and activate complement, damaging the thyroid
BReceptor stimulation: antibodies mimic the natural ligand (TSH) and activate the receptor constitutively
CReceptor blockade: antibodies occupy the TSH receptor and prevent normal signaling
DCytotoxic T cell activation: antibodies mark thyroid cells for destruction by CD8+ T cells
Anti-TSH receptor antibodies in Graves' disease bind the TSH receptor and mimic the effect of TSH itself, driving continuous thyroid hormone synthesis. This is the stimulating-receptor mechanism — distinct from blocking (as in myasthenia gravis, where anti-AChR antibodies block acetylcholine binding) and immune complex deposition (as in lupus nephritis, where anti-dsDNA antibodies form complexes that deposit in the kidney and activate complement).
Question 3 True / False
Autoantibodies found in a patient's serum prove that tissue damage is occurring through an autoimmune mechanism.
TTrue
FFalse
Answer: False
Autoantibodies are not always pathogenic — they can be bystanders produced during bystander B cell activation without directly causing tissue damage. The presence of autoantibodies must be interpreted in clinical context: some are diagnostic markers (anti-CCP in RA, anti-dsDNA in lupus) but may not be the primary effectors of damage in all cases. Pathogenicity depends on the target, titer, antibody class, and whether the target is accessible.
Question 4 True / False
In many established autoimmune diseases, the immunological imbalance tends to sustain itself even after the original trigger has resolved, because IL-6 simultaneously promotes Th17 differentiation and inhibits Treg differentiation.
TTrue
FFalse
Answer: True
IL-6 creates a self-reinforcing loop: it drives naïve T cells toward the Th17 phenotype (pro-inflammatory, IL-17-producing) while suppressing Treg differentiation (anti-inflammatory). This means the cytokine environment generated by autoimmune inflammation actively undermines the suppressive mechanisms that would normally terminate the response. This is why autoimmune diseases tend to be chronic or relapsing-remitting rather than self-limited, even when the original triggering antigen is gone.
Question 5 Short Answer
Why is HLA association with an autoimmune disease not sufficient to cause disease on its own? What additional steps or failures are required?
Think about your answer, then reveal below.
Model answer: HLA alleles influence which self-peptides are presented during thymic selection and which peripheral antigens trigger responses — setting susceptibility. But disease also requires failure of peripheral tolerance mechanisms (anergy, Treg suppression), and typically an environmental trigger such as infection that provides molecular mimicry or bystander activation to expand autoreactive clones. All three layers — genetic susceptibility, tolerance failure, and environmental trigger — must converge.
This multi-hit model explains why concordance in identical twins is ~30–50% rather than ~100%. It also explains why autoimmune diseases cluster geographically and temporally (infection outbreaks preceding peaks in certain autoimmune diagnoses) and why the same HLA allele raises risk for disease without guaranteeing it. Therapeutically, this means that interrupting any one of the three converging pathways — blocking T cell co-stimulation, restoring Treg function, or reducing inflammatory cytokines — can break the cycle even without removing the genetic risk.