A patient carries HLA-DR4, a risk allele for rheumatoid arthritis. At age 40, following a bacterial infection, she develops joint inflammation and tests positive for anti-citrullinated protein antibodies. Her identical twin, also carrying HLA-DR4, remains healthy at age 55. What best explains this discrepancy?
AHLA-DR4 does not actually increase rheumatoid arthritis risk — it is an unrelated genetic marker
BThe twin is protected by stronger central tolerance that more efficiently deletes self-reactive T cells
CAutoimmune disease requires multiple converging factors — genetic predisposition alone is insufficient; an environmental trigger and failure of peripheral tolerance must also occur
DThe patient's central tolerance is fully intact; peripheral tolerance is her only deficiency
This is the multi-hit model of autoimmunity in practice. HLA-DR4 increases risk but is not deterministic — the majority of HLA-DR4 carriers never develop rheumatoid arthritis. The patient required three converging factors: the genetic predisposition (HLA-DR4), an environmental trigger (the bacterial infection, potentially acting through molecular mimicry or releasing sequestered antigens), and a failure of peripheral tolerance (Treg dysfunction or anergy breakdown) that allowed self-reactive cells to persist and expand. The twin, sharing the same genetic risk, did not encounter the necessary environmental and regulatory co-factors.
Question 2 Multiple Choice
Which of the following best describes the mechanism of molecular mimicry in autoimmunity?
APathogens insert foreign DNA into host immune cells, reprogramming them to attack self-tissue
BImmune responses generated against a pathogen cross-react with structurally similar self-proteins, triggering persistent autoimmune attack after the infection resolves
CSelf-reactive T cells that escaped thymic deletion are activated directly by inflammatory cytokines without any pathogen involvement
DRegulatory T cells misidentify self-antigens as foreign and activate autoreactive B cells
Molecular mimicry occurs when pathogen proteins share structural (sequence or conformational) similarity with self-proteins. The adaptive immune response generated during infection — T cells and antibodies that effectively clear the pathogen — cross-reacts with the mimicked self-antigen. After the pathogen is cleared, the self-reactive response persists, producing sustained autoimmune damage. Rheumatic fever is the textbook example: antibodies against streptococcal M protein cross-react with cardiac myosin, causing valve damage. The mechanism explains why infections can trigger autoimmunity in genetically susceptible individuals.
Question 3 True / False
Central tolerance in the thymus eliminates most self-reactive T cells, making peripheral tolerance merely a redundant backup that is rarely needed in healthy individuals.
TTrue
FFalse
Answer: False
Central tolerance is imperfect by design. Some self-antigens are not expressed in the thymus (tissue-specific antigens may be absent or present at too low a level to drive deletion), and the deletion threshold is not absolute — some weakly self-reactive cells survive. Peripheral tolerance is essential and active: anergy, Treg suppression, and activation-induced cell death continuously manage the self-reactive cells that escaped central deletion. IPEX syndrome — caused by FoxP3 mutations that abolish Treg function — produces severe, fatal multi-organ autoimmunity even with intact central tolerance, demonstrating that peripheral tolerance is not merely backup.
Question 4 True / False
Organ-specific and systemic autoimmune diseases differ primarily in which type of immune effector mediates the damage (antibody-mediated vs. T cell-mediated), not in the scope of the target.
TTrue
FFalse
Answer: False
The primary distinction is the scope of the target, not the effector type. Both organ-specific diseases (Type 1 diabetes, Hashimoto's thyroiditis) and systemic diseases (SLE, rheumatoid arthritis) can involve both autoreactive T cells and autoantibodies — the mechanisms overlap. What differs is whether the immune attack is directed at tissue-specific antigens (pancreatic β cells, thyroid tissue) or at ubiquitous antigens found throughout the body (DNA, nuclear proteins, joint-lining proteins). Systemic diseases cause multi-organ damage precisely because their autoantibody targets are present everywhere.
Question 5 Short Answer
Why is it accurate to say that the same features of the adaptive immune system that make it effective against pathogens also make it capable of causing severe autoimmune disease?
Think about your answer, then reveal below.
Model answer: The adaptive immune system's power comes from its exquisite specificity and clonal expansion — it generates lymphocytes with precise, high-affinity recognition of a target antigen and then amplifies those cells massively. This specificity is generated randomly (through V(D)J recombination) and then shaped by selection to be useful against foreign antigens. But the same mechanism that generates a powerful response against a pathogen can produce self-reactive cells if the selection process fails. When an autoreactive T cell or B cell escapes tolerance and encounters its self-antigen, it uses the same mechanisms of activation, proliferation, and effector function — cytotoxicity, antibody production, inflammatory signaling — that normally eliminate pathogens. The disease is caused by precision-targeted immune destruction, which is why autoimmune diseases are often severe and tissue-specific.
Tolerance mechanisms exist precisely because the adaptive immune system's random receptor generation inevitably produces some self-reactive receptors. Central and peripheral tolerance are the price paid for having a flexible, broadly reactive immune system. When those checkpoints fail, the system's adaptive power — its ability to mount a focused, sustained, high-affinity attack — becomes directed inward with the same intensity it normally reserves for pathogens.