Signal 1 plus signal 2 turns a T cell on. Drop signal 2 and the cell goes anergic. Tumors and immunotherapies exploit that same switch. Here is how to never mix it up.
A 64-year-old woman with long-standing seropositive rheumatoid arthritis, still symptomatic on methotrexate and a TNF inhibitor, enrolls in a trial of a fusion protein that mimics CTLA-4 and binds CD80 / CD86 on antigen-presenting cells. Eight weeks in, her morning stiffness shortens, the swollen joint count falls, and both CRP and interleukin-2 decline.
Which immune mechanism best explains her improvement?
A CTLA-4 fusion protein (abatacept) mops up B7 (CD80 / CD86), so CD28 never gets its handshake. That strips away signal 2, the costimulatory signal. A T cell that sees antigen through signal 1 (TCR plus MHC) but never gets signal 2 does not activate. It goes anergic: alive, but switched off, and refractory to later full stimulation. Fewer activated T cells in the joint means less IL-2, less inflammation, fewer swollen joints.
Why the distractors fail: complement and immune-complex clearance are humoral effects, not how a CTLA-4 fusion protein works. Negative selection in the thymus is central tolerance, a developmental event that finished decades ago, not something a drug induces in an adult. And B-cell immunoglobulin output is downstream of T-cell help, not the direct target.
Rule: signal 1 without signal 2 equals anergy. Abatacept steals signal 2.
Concept 1 of 5
The Two-Signal Model
A naive T cell needs two handshakes to wake up. Toggle the signals, predict the fate, then run it.
CD28 (the GO pedal)
CTLA-4 (the BRAKE)
CD28
The activating costimulatory receptor on the T cell
Lives on
Naive T cells (constitutive)
Binds
B7 (CD80 / CD86) on the APC
Effect
Delivers signal 2: produces IL-2, drives clonal expansion
Without it
Signal 1 alone → T cell goes anergic
Board Trap
CD28 is the accelerator. If a drug blocks CD28 meeting B7 (abatacept), the T cell sees its antigen but cannot get the second handshake, so it shuts down instead of attacking. Signal 1 without signal 2 equals anergy.
CTLA-4
The inhibitory checkpoint, also a B7 receptor
Lives on
T cells; constitutive on Tregs, induced on activated T cells
Binds
Same B7 (CD80 / CD86) as CD28
Effect
Sends an OFF signal, halts IL-2 and clonal expansion
Affinity
Outcompetes CD28 for B7 (higher affinity)
Board Trap
CTLA-4 and CD28 bind the SAME ligand (B7), but CTLA-4 has higher affinity and wins the competition, shutting activation down. Ipilimumab blocks CTLA-4 to release the brake at the priming stage in the lymph node. CTLA-4 is the brake; CD28 is the gas. Same ligand, opposite effect.
Before you run it: with the signals YOU toggled, what happens to this T cell?
Concept 2 of 5
Central vs Peripheral Tolerance
Two checkpoints catch self-reactive lymphocytes. Sort each mechanism to where it actually happens.
Malar rash of SLE · tolerance failure in real life
When tolerance fails, autoreactive T and B cells escape and attack self tissue. A lupus malar rash is one face of that failure.
1
A self-reactive thymocyte that strongly binds self antigen on MHC in the thymic medulla is deleted. Which layer of tolerance is this?
2
Correct. That is negative selection in the thymus, driven by the AIRE transcription factor. Now: a mature T cell in a lymph node sees antigen but gets no B7 costimulation. What is its fate?
Concept 3 of 5
Checkpoint Cancer Immunotherapy
Tumors wear PD-L1 to exhaust the T cells sent to kill them. Watch the evasion, then watch the antibody cut the brake.
Malignant melanoma · the PD-1 tumor
Vitiligo on anti-PD-1 · brakes off everywhere
Nadia, 68, has unresectable melanoma. Her CD8 T cells reached the tumor but cannot kill it. Press fire to see why.
PD-1 blocked. The exhausted T cell wakes up and destroys the tumor.
Pattern
Tumor upregulates PD-L1 → engages PD-1 on the T cell → T-cell exhaustion (anergy at the tumor bed). Block PD-1 and the brake lifts: the cytotoxic T cell releases perforin and granzyme and kills the tumor.
Drugs
anti-PD-1: pembrolizumab, nivolumab, cemiplimab. anti-PD-L1: atezolizumab, durvalumab, avelumab. anti-CTLA-4: ipilimumab (releases the brake at priming in the lymph node).
Pearl
Releasing tolerance brakes causes immune-related adverse events: dermatitis, colitis, hepatitis, pneumonitis, thyroiditis, hypophysitis. Vitiligo in a melanoma patient on anti-PD-1 is a good prognostic sign: T cells are recognizing pigment antigens shared by tumor and normal melanocytes.
Concept 4 of 5
Fas, AICD & the Tolerance Gene Lineup
Each layer of tolerance has a signature gene. Break the gene, get a disease. Tap a card to flip.
☣
Fas (CD95) / FasL
Peripheral: activation-induced cell death
Defect → ALPS
tap to flip →
Why ALPS happens
Fas binding FasL triggers the caspase cascade that deletes activated, repeatedly-stimulated lymphocytes (AICD). A germline Fas mutation prevents that cleanup. Autoreactive and activated cells survive and accumulate: chronic lymphadenopathy, splenomegaly, autoimmune cytopenias, SLE-like features, and double-negative T cells (CD3+, CD4-, CD8-). Fas broken equals lymphocytes that refuse to die.
👥
FoxP3
Peripheral: regulatory T cells
Defect → IPEX
tap to flip →
Why IPEX happens
FoxP3 is the master transcription factor of Tregs (CD4+, CD25+, CTLA-4+, making IL-10 and TGF-beta). Lose FoxP3 and Tregs vanish. Result is IPEX: Immunodysregulation, Polyendocrinopathy, Enteropathy, X-linked. Early-onset type 1 diabetes, severe diarrhea, eczema. FoxP3 gone equals no brakes on autoimmunity.
🧬
AIRE
Central: negative selection
Defect → APS-1
tap to flip →
Why APS-1 happens
AIRE lets medullary thymic epithelial cells express peripheral-tissue antigens so developing T cells can be negatively selected against them. Without AIRE, self-reactive clones escape central tolerance. Result is autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APS-1 / APECED): mucocutaneous candidiasis, hypoparathyroidism, adrenal insufficiency. AIRE broken equals central tolerance that never screened self.
🔌
CTLA-4 / CD28-B7
Peripheral: costimulation / anergy
Drug target → abatacept, ipilimumab
tap to flip →
Two drugs, one axis
Block CD28 from meeting B7 and you force anergy (abatacept, CTLA-4 fusion protein, for RA). Block CTLA-4 instead and you remove the brake on T-cell priming (ipilimumab, for melanoma). Same ligand, opposite goals. CTLA-4 haploinsufficiency in people (not a drug) causes spontaneous autoimmunity and lymphoproliferation. Same axis, two directions: dampen to treat autoimmunity, release to treat cancer.
Concept 5 of 5
Memory Hooks
Tap a card to unblur the rule. Test yourself first.
💡
Signal 1 vs signal 2
Signal 1 (TCR + MHC) without signal 2 (CD28 + B7) equals anergy. Abatacept steals signal 2 to treat RA.
tap to reveal
⚠
CD28 vs CTLA-4
Both bind B7. CD28 is the gas (GO), CTLA-4 is the brake (STOP). CTLA-4 has higher affinity and wins. Ipilimumab cuts the brake.
tap to reveal
🎯
PD-1 in one line
Tumor PD-L1 hits PD-1 and exhausts the T cell. Pembrolizumab and nivolumab block PD-1 and wake the cell up.
tap to reveal
💀
Fas and ALPS
Fas (CD95) + FasL equals apoptosis of activated lymphocytes. Broken Fas equals ALPS: big nodes, big spleen, SLE-like, double-negative T cells.
Cutting tolerance brakes causes colitis, dermatitis, hepatitis, pneumonitis, thyroiditis, hypophysitis. Vitiligo in melanoma on anti-PD-1 is a good sign.
tap to reveal
Prove it
Board-Style Walkthrough
Original board-style vignettes. Shuffled, never-repeat until the bank is exhausted. Cross out (right-click or long-press) and highlight (double-click or double-tap) before you answer. Clues glow after you commit.
Tip: right-click or long-press an option to cross it out; double-click or double-tap to highlight.
