Step by Step
CT
Central Tolerance — deleting self-reactive cells before they're ever released
Central tolerance happens during lymphocyte development. In the thymus, T cells that react too strongly to self-antigens are deleted — a process called negative selection, guided by the AIRE gene, which allows the thymus to display a broad sample of the body's own proteins for this screening. Autoreactive B cells undergo a similar deletion process in the bone marrow.
Mutations in the AIRE gene impair this thymic screening process and cause APECED syndrome, a rare condition marked by multiple autoimmune diseases developing simultaneously, since self-reactive T cells that should have been deleted are instead released into circulation.
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Peripheral Tolerance — a backup system for cells that slip through
Central tolerance isn't perfect — some self-reactive lymphocytes do slip through. Peripheral tolerance is the backup system: regulatory T cells (Tregs, marked as CD4+CD25+FoxP3+) actively suppress autoreactive cells, and mechanisms like anergy and activation-induced cell death (AICD) provide additional layers of control outside the thymus and bone marrow.
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Failure Mechanisms — how tolerance actually breaks down
Autoimmunity results when both these tolerance layers fail. Molecular mimicry occurs when a pathogen's proteins closely resemble the body's own proteins, so an immune response against the pathogen accidentally also targets self-tissue. Bystander activation and Treg deficiency are other common routes to the same outcome — self-reactive cells escaping suppression.
Rheumatic fever develops when antibodies produced against Group A Streptococcus cross-react with cardiac tissue, due to molecular mimicry between streptococcal proteins and proteins in heart valve tissue — leading to autoimmune damage to the heart.
Applied Walkthrough
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A patient develops joint pain and cardiac symptoms several weeks after a streptococcal throat infection.
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Ask: how could an infection cause the body to attack its own heart tissue? Through molecular mimicry — the antibodies generated against streptococcal M protein happen to cross-react with proteins in cardiac tissue, since the two protein structures closely resemble one another.
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Contrast: this is different from a scenario like Type 1 diabetes, where autoimmune destruction of pancreatic beta cells isn't necessarily triggered by a specific cross-reactive infection, but more often reflects a breakdown in central and peripheral tolerance checkpoints more broadly (with genetic predisposition via HLA-DR3/4).
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The molecular mimicry scenario (rheumatic fever) versus a broader tolerance-checkpoint failure (T1DM) illustrates that 'autoimmunity' isn't one single mechanism — exams expect you to identify which failure pathway fits a given clinical picture.
Exam Application
Exams test the distinction between central tolerance (thymic/bone marrow deletion, AIRE gene, and what happens when AIRE fails — APECED syndrome) and peripheral tolerance (Tregs, anergy, AICD), plus specific failure mechanisms like molecular mimicry, using classic examples: rheumatic fever, T1DM, SLE, RA, and Graves' disease.
⚠ Common Trap
The most common trap is treating 'autoimmunity' as a single mechanism rather than recognizing it results from a failure at one of several distinct checkpoints — central tolerance, peripheral tolerance, or a specific triggering event like molecular mimicry. Exam questions often expect you to identify which checkpoint failed in a given scenario, not just recognize that autoimmunity occurred.
✓ Quick Self-Check
1. Where does central tolerance occur, and what gene helps drive it in the thymus?
In the thymus (for T cells) and bone marrow (for B cells); the AIRE gene enables the thymus to display self-antigens for negative selection screening.
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2. What condition results from a mutation in the AIRE gene?
APECED syndrome, marked by multiple simultaneous autoimmune diseases due to failed thymic deletion of self-reactive T cells.
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3. What cells provide peripheral tolerance, and what marker identifies them?
Regulatory T cells (Tregs), marked as CD4+CD25+FoxP3+.
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4. What is molecular mimicry, and give a clinical example.
When a pathogen's proteins closely resemble the body's own proteins, causing an immune response against the pathogen to also target self-tissue; example: rheumatic fever, where anti-streptococcal antibodies cross-react with cardiac tissue.
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5. Name two other classic examples of autoimmune disease mentioned besides rheumatic fever.
Type 1 diabetes mellitus (autoimmune destruction of pancreatic beta cells) and Graves' disease (autoimmune stimulation via anti-TSH-receptor antibodies), among others like SLE and RA.
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