Mechanisms of Tolerance Breakdown: A High-Yield Immunology Review
Immune tolerance is maintained centrally (thymic negative selection of T cells via AIRE-presented self-antigens; B-cell receptor editing and deletion in the marrow) and peripherally (Tregs, anergy, and deletion by activation-induced cell death). Autoimmunity results when these safeguards fail and self-reactive lymphocytes escape control. This review frames the distinct pathways by which self-tolerance breaks down and pairs each with its classic example.
Central and Peripheral Tolerance (The Framework)
Central tolerance eliminates self-reactive cells during development: T cells undergo negative selection in the thymus, where AIRE presents self-antigens, while B cells undergo receptor editing and deletion in the bone marrow. Peripheral tolerance provides backup for cells that escape: regulatory T cells (Tregs) suppress autoreactive cells, anergy renders a T cell unresponsive when it sees antigen without costimulation, and deletion occurs when repeated stimulation triggers activation-induced cell death. Tolerance breakdown occurs when one or more of these checkpoints fails.
Molecular Mimicry
A foreign antigen shares structural similarity with a self-antigen, so an immune response mounted against the microbe cross-reacts with host tissue. The classic example is rheumatic fever, in which streptococcal M protein resembles cardiac myosin, leading antibodies raised against strep to attack the heart. Note that the resulting cardiac injury operates through Type II (antibody-mediated) hypersensitivity.
Epitope Spreading
An immune response that begins against a single epitope broadens over time to target additional epitopes—on the same antigen or on other self-antigens released during tissue damage. This diversification amplifies and perpetuates autoimmune injury even after the initial trigger is gone.
Bystander Activation
Inflammation (from infection or tissue injury) releases self-antigens and creates a proinflammatory environment that activates nearby autoreactive lymphocytes non-specifically. Self-antigens normally ignored become visible and immunogenic in this inflammatory milieu.
Failure of Tregs
Loss of regulatory T-cell function removes a key peripheral tolerance checkpoint, allowing autoreactive cells to proceed unchecked. The prototype is IPEX syndrome, caused by a FOXP3 mutation—FOXP3 is the master transcription factor required for Treg development and function.
Sequestered Antigen Release
Some self-antigens reside in immune-privileged sites and are never presented to the developing immune system, so no tolerance is established against them. Trauma that breaches the barrier exposes these hidden antigens and triggers an immune attack. The classic example is sympathetic ophthalmia, in which eye trauma releases sequestered ocular antigens and provokes an immune response against the uninjured eye.
High-yield
- Molecular mimicry → rheumatic fever: strep M protein resembles cardiac myosin.
- IPEX syndrome = FOXP3 mutation → failed Tregs → multi-organ autoimmunity.
- Sympathetic ophthalmia = sequestered antigen release after eye trauma affecting the fellow eye.
- AIRE mediates thymic presentation of self-antigens for T-cell negative selection (central tolerance).
- B-cell central tolerance = receptor editing and deletion in bone marrow.
- Peripheral tolerance triad: Tregs, anergy (antigen without costimulation), and deletion (activation-induced cell death).
- Epitope spreading explains why autoimmune responses broaden and persist over time.
- Bystander activation: inflammation releases self-antigens and activates autoreactive cells non-specifically.
Pitfalls
- Confusing molecular mimicry (cross-reactive shared epitope) with bystander activation (non-specific activation via inflammatory release of self-antigens).
- Assuming tolerance is only central—peripheral mechanisms (Tregs, anergy, deletion) are equally testable checkpoints.
- Forgetting that FOXP3 is the defect in IPEX; it is Treg failure, not a mimicry or sequestration mechanism.
- Mixing up the tolerance mechanism (why autoimmunity starts) with the hypersensitivity type (how tissue is damaged)—e.g., rheumatic fever is molecular mimicry that causes Type II injury.
- Overlooking immune privilege: sequestered antigens never induced tolerance because they were never presented, so trauma—not mimicry—triggers the disease.
Clinical pearls
- When trauma to one eye later inflames the other eye, think sympathetic ophthalmia from sequestered antigen release.
- A male infant (X-linked) with intractable diarrhea, endocrinopathy, and eczema-like autoimmunity suggests IPEX from FOXP3 mutation.
- Post-streptococcal cardiac damage is the board's favorite illustration of molecular mimicry.
- AIRE defects break central T-cell tolerance by failing to present self-antigens in the thymus.
Frequently asked
What is the difference between central and peripheral tolerance?
Central tolerance eliminates self-reactive cells during development—T cells by thymic negative selection (AIRE presents self-antigens) and B cells by receptor editing and deletion in the marrow. Peripheral tolerance controls cells that escape, using Tregs, anergy, and deletion via activation-induced cell death.
How does molecular mimicry cause autoimmunity?
A microbial antigen structurally resembles a self-antigen, so the immune response against the pathogen cross-reacts with host tissue. Rheumatic fever is the classic example—streptococcal M protein resembles cardiac myosin.
What is epitope spreading?
An immune response that starts against one epitope broadens over time to target additional epitopes, on the same or other self-antigens, amplifying and perpetuating autoimmune damage.
What causes IPEX syndrome and which tolerance mechanism fails?
IPEX results from a FOXP3 mutation, which impairs regulatory T cells. It is a failure of peripheral tolerance because Tregs can no longer suppress autoreactive cells.
Why does eye trauma cause sympathetic ophthalmia?
Ocular antigens are sequestered in an immune-privileged site, so tolerance to them was never established. Trauma releases these hidden self-antigens, triggering an immune attack that can involve the uninjured eye.
What is bystander activation?
Inflammation from infection or injury releases self-antigens and creates a proinflammatory environment that non-specifically activates nearby autoreactive lymphocytes that would otherwise stay quiet.
What are the three mechanisms of peripheral tolerance?
Regulatory T cells suppressing autoreactive cells, anergy (a T cell seeing antigen without costimulation becomes unresponsive), and deletion (repeated stimulation leads to activation-induced cell death).
Does molecular mimicry describe how tissue is damaged?
No—molecular mimicry describes why tolerance broke down. The actual tissue injury proceeds through a hypersensitivity pathway; in rheumatic fever, the cross-reactive antibodies cause Type II (antibody-mediated) damage.
Turn this into reasoning you can use on exam day — practice Mechanisms of Tolerance Breakdown on branching cases where your decisions shape the patient.