T Lymphocytes
Growing up in the thymus ★★★
A T cell is made in the bone marrow but educated in the thymus, and the education is brutal: the overwhelming majority of thymocytes die there. Understanding why they die is understanding the whole design, because the thymus has to solve two problems at once. It must produce cells that can see antigen on your own MHC molecules, and it must destroy any cell that reacts to you.
| Detail | |
|---|---|
| Stages | Double-negative (DN) CD4⁻CD8⁻ → double-positive (DP) CD4⁺CD8⁺ → single-positive (SP) CD4⁺CD8⁻ or CD4⁻CD8⁺ |
| Events | Generation of the TCR · positive selection · negative selection |
| Results | Expression of a TCR · MHC restriction · self-tolerance |
Generating the TCR
In the thymic cortex, during the DN→DP transition, the TCRβ gene rearranges first and the cell expresses a pre-TCR (pTα paired with TCRβ). Then the TCRα gene rearranges and a complete αβTCR appears at the DP stage. The result is a functional receptor and an enormous diversity of TCR repertoire — the library of random keys from Unit 1's clonal selection.
- Where do T cells mature? → The thymus
- Name the three developmental stages. → Double-negative → double-positive → single-positive
- Which TCR chain rearranges first? → β, giving the pre-TCR; then α
- What are the three results of thymic development? → TCR expression, MHC restriction, self-tolerance
Positive and negative selection ★★★
Both selections show the thymocyte the same thing — self-peptide bound to self-MHC — and ask a question about affinity. But they want opposite answers, and that is the key to keeping them straight.
Positive selection asks: can you see MHC at all? Bind with proper affinity and you live; fail to bind and you are useless, so you die. Negative selection asks: do you react to self too strongly? Bind with high affinity and you are dangerous, so you die.
So the survivor is a Goldilocks cell: it binds self-MHC enough to be useful but not enough to be autoreactive. Positive selection gives MHC restriction; negative selection gives self-tolerance.
| Positive selection | Negative selection | |
|---|---|---|
| Location | Thymic cortex | Cortico-medullary junction and medulla |
| Stage | DP | SP |
| Presented by | Thymic epithelial cells | Thymic dendritic cells and macrophages |
| Survive if… | Recognise self-peptide–MHC with proper (moderate) affinity | Fail to recognise, or recognise with low affinity |
| Die if… | Unable to recognise, or recognise with high affinity | Recognise self-peptide–MHC with high affinity |
| Result | Self-MHC restriction | Self-immune tolerance |
Positive selection also decides the lineage. A DP cell whose TCR engages MHC class I becomes CD3⁺CD4⁻CD8⁺; one that engages MHC class II becomes CD3⁺CD4⁺CD8⁻. The co-receptor that is kept is the one that matched — which is why CD8 T cells are class-I restricted and CD4 T cells class-II restricted for the rest of their lives.
- Where does positive selection occur, and at which stage? → Thymic cortex, DP stage
- Where does negative selection occur? → Cortico-medullary junction and medulla, SP stage
- Which cells present in each? → Positive: thymic epithelial cells. Negative: dendritic cells and macrophages
- What does positive selection confer? → Self-MHC restriction
- What does negative selection confer? → Self-tolerance
- A DP cell recognising MHC class II becomes? → CD4⁺CD8⁻ single positive
The TCR–CD3 complex ★★★
The T-cell receptor has a problem: it binds antigen but has almost no cytoplasmic tail, so it cannot signal. The solution is a partnership. TCR binds; CD3 signals. They travel together as the TCR–CD3 complex, and each is a lineage-specific marker of T cells.
| TCR | CD3 | |
|---|---|---|
| Function | Antigen binding | Signal transduction from the TCR |
| Also | Lineage-specific marker | Lineage-specific marker |
| Types / parts | TCRαβ (95–99%) and TCRγδ (1–5%) | Cytoplasmic tails carry ITAM motifs |
A short amino-acid sequence in the cytoplasmic tails of CD3 containing two tyrosine residues about 10 residues apart, which become phosphorylated on activation and are responsible for signal transduction.
The fact that a given T cell will recognise a peptide antigen only when it is bound to a particular MHC molecule — normally, self-MHC. It is acquired during positive selection, so the MHC haplotype of the thymus in which a T cell develops determines its restriction. CD8⁺ CTLs are restricted by MHC class I; CD4⁺ helper T cells by MHC class II.
Notice the pattern: an antigen receptor that binds but cannot signal, paired with an invariant chain that signals but cannot bind. You meet it again immediately in Unit 8 — the BCR is membrane immunoglobulin paired with Igα/Igβ, which carry ITAMs of their own. Two receptors, one design. And the inhibitory receptors in §6 use the mirror-image motif, ITIM, where the I is for inhibition.
- What does TCR do, and what does CD3 do? → TCR binds antigen; CD3 transduces the signal
- What proportion of T cells are αβ? → 95–99%; γδ are 1–5%
- What is an ITAM and where is it? → A two-tyrosine activation motif in the CD3 cytoplasmic tails
- Define MHC restriction. → A T cell recognises peptide only when bound to a particular (self) MHC molecule
- Which selection confers MHC restriction? → Positive selection
- CD8 is restricted by which MHC class? → Class I. CD4 by class II
CD4 and CD8 co-receptors ★★★
Mature αβ T cells express either CD4 or CD8, never both. Their job is to grip the MHC molecule alongside the TCR — which is why Unit 6 classified them as adhesion molecules. Crucially, each binds a non-polymorphic part of its MHC class, away from the peptide groove, so the grip is the same regardless of which peptide is being shown.
| CD4 | CD8 | |
|---|---|---|
| Structure | Monomer | Heterodimer |
| Expression | 60–65% of αβ T cells; some NKT cells; lower levels on mononuclear phagocytes and some DCs | 30–35% of αβ T cells; some γδ T cells |
| Binds | The β2 domain of MHC class II | The α3 domain of MHC class I |
| Function | Increases the sensitivity of the antigen receptor; participates in activation signalling | Same |
| Also | Receptor for HIV gp120 | — |
HIV's envelope protein gp120 binds CD4. That single fact explains the shape of the disease: the virus enters through the molecule that defines the helper T cell, so it depletes precisely the population that coordinates both arms of adaptive immunity. And even before the cell dies, a CD4 molecule occupied by gp120 cannot engage MHC class II on an APC — so helper function is impaired before helper cells are lost. Hence the pattern of AIDS: not one missing defence, but a general collapse in everything that needed T-cell help.
- Can a mature αβ T cell express both CD4 and CD8? → No — either one or the other
- CD4 structure and MHC target? → Monomer; binds the β2 domain of MHC class II
- CD8 structure and MHC target? → Heterodimer; binds the α3 domain of MHC class I
- What proportion of αβ T cells carry each? → CD4 60–65%, CD8 30–35%
- Which co-receptor is the HIV receptor? → CD4, binding gp120
The two-signal rule ★★★
A T cell cannot be activated by a single signal. It needs two, and the requirement is one of the most important safety features in immunology.
| Signal | Delivered by | Partner |
|---|---|---|
| Signal 1 — the TCR signal | TCR–CD3 complex recognising the antigen–MHC complex, with CD4 or CD8 as co-receptor | peptide–MHC on the APC |
| Signal 2 — the co-stimulatory signal | CD28 on the T cell | B7.1 (CD80) / B7.2 (CD86) on the APC |
| Signal 3 | Local cytokines — direct differentiation into distinct effector types | e.g. IL-12 → Th1, IL-4 → Th2 (Unit 5) |
Signal 1 says “I have found my antigen.” Signal 2 says “and this antigen is genuinely dangerous.” B7 appears on an APC only when it has been activated by encountering a pathogen — so signal 2 is effectively the innate immune system vouching for the threat.
Now consider a T cell that meets its antigen on a resting cell with no B7. It gets signal 1 alone — and instead of activating, it is switched off, becoming anergic. That is not a failure; it is a second layer of tolerance, operating in the periphery on any self-reactive cell that slipped past negative selection in the thymus. Requiring two signals means that recognising something is not sufficient reason to attack it.
- What are the two signals for T-cell activation? → TCR–CD3 with peptide–MHC; CD28 with B7
- What is signal 3? → Local cytokines, directing the effector type
- Where does B7 come from? → Activated DCs, macrophages and B cells
- What happens if a T cell gets signal 1 without signal 2? → It becomes anergic — peripheral tolerance
Co-stimulatory molecules — accelerator and brake ★★★
The deck's own metaphor is the right one. Some surface molecules act as an accelerator and some as a brake, and together they are called immune checkpoints. Learn them as two lists, because the exam asks which one down-regulates.
| Molecule | On | Ligand | Effect |
|---|---|---|---|
| CD28 — accelerator | 90% of CD4⁺, 50% of CD8⁺ T cells (homodimer) | B7.1 (CD80) / B7.2 (CD86) | Activation — signal 2 |
| CTLA-4 (CD152) — brake | Induced on activation; absent from resting cells | B7.1 / B7.2 — the same ligands as CD28 | Inhibition, via an ITIM motif |
| ICOS — accelerator | Activated T cells | ICOSL (B7-H2) | Works after CD28; promotes proliferation and regulates cytokine production |
| PD-1 — brake | Activated T cells | PD-L1 and PD-L2 | Reduces T-cell proliferation and IFN-γ/IL-2 secretion; inhibits B-cell proliferation |
| CD40L (CD154) | Activated T cells | CD40 on APC / B cell | Activates APCs; delivers the co-stimulatory signal for B-cell activation |
| CD2 (LFA-2) | 95% of mature T cells, some NK cells | LFA-3 (CD58) | Adhesion and co-stimulation; the sheep-erythrocyte E-rosette receptor |
| LFA-1 / ICAM-1 | T cells and APCs | ICAM-1 / LFA-1 | Intercellular adhesion (Unit 6) |
Tumours exploit this system. Many express high levels of PD-L1, which engages PD-1 on tumour-infiltrating T cells and switches them off — the cancer is pulling the immune system's own brake. Checkpoint inhibitors are monoclonal antibodies that block PD-1/PD-L1 or CTLA-4, releasing that brake and restoring T-cell activity against the tumour.
This is the single biggest change in cancer therapy in decades, and it came directly out of the basic immunology in this table. It also explains the characteristic toxicity: take the brakes off T cells and you get autoimmune side effects — colitis, thyroiditis, hepatitis. You meet these again in Unit 17.
- Name two positive co-stimulatory molecules. → CD28 and ICOS
- Name two negative ones (coinhibitory). → CTLA-4 and PD-1
- Which molecule down-regulates T-cell activation? → CTLA-4 — this is MCQ Q16
- Why is CTLA-4 confusable with CD28? → They bind the same ligands, B7.1 and B7.2
- What motif makes CTLA-4 inhibitory? → ITIM
- CD40L is on which cell, and binds what? → Activated T cells; binds CD40 on APC/B cells
- What is CD2's ligand? → LFA-3 (CD58)
T-cell subsets ★★
T cells can be divided four different ways, and the deck lists all four. Which division you use depends on the question being asked.
- By response to antigen — naïve, effector, memory
- By TCR — αβ or γδ
- By co-receptor — CD4⁺ or CD8⁺
- By function — helper (Th), cytotoxic (CTL/Tc), regulatory (Treg)
| αβ T cells | γδ T cells | |
|---|---|---|
| Proportion | 95–99% | 1–5% |
| TCR diversity | Enormous | Limited |
| MHC restriction | Yes | No |
| Distribution | Secondary lymphoid tissue | Skin and mucosal tissue |
| Classed as | Adaptive | Innate immune cells |
The CD4⁺ subsets
Th1 and Th2 are taught in Unit 5 §6, with the cytokine profiles and the True/False trap that turns on them. Three further CD4⁺ subsets belong here.
| Subset | Markers / signature | Function |
|---|---|---|
| Treg (regulatory T cells) | CD4⁺ CD25⁺, master transcription factor FoxP3 | Modulate the immune system, maintain tolerance to self-antigens and prevent autoimmune disease. Inhibit via IL-10 and TGF-β, and can kill activated T cells directly. Thymic Treg develop in the thymus; peripheral Treg differentiate in the periphery |
| Th17 | Produce IL-17, IL-17F, IL-22; also IL-21 | Induce inflammation to destroy extracellular bacteria and fungi; contribute to several inflammatory diseases |
| Tfh (follicular helper) | Express CXCR5; secrete IL-21 | Migrate into B-cell follicles in secondary lymphoid organs and facilitate the germinal centre reaction |
Negative selection in the thymus deletes strongly self-reactive cells — but it cannot possibly show every thymocyte every self antigen in the body. Some autoreactive cells inevitably escape. Treg cells are the standing peripheral answer to that: a population whose entire job is suppressing responses that should not be happening. Lose FoxP3 and you get IPEX syndrome — overwhelming, multi-organ autoimmunity from birth, which is what tolerance failing completely looks like.
- Four ways of classifying T cells? → By antigen response, TCR type, co-receptor, function
- αβ vs γδ — which is MHC-restricted? → αβ. γδ is not, and counts as innate
- Where do γδ T cells sit? → Skin and mucosal tissue
- Treg markers? → CD4⁺CD25⁺, with FoxP3 as master regulator
- How do Treg suppress? → IL-10 and TGF-β, and by killing activated T cells directly
- What do Th17 cells make, and against what? → IL-17, IL-17F, IL-22 — against extracellular bacteria and fungi
- What do Tfh cells do? → Enter B-cell follicles and drive the germinal centre reaction
CD8⁺ cytotoxic T cells ★★★
The CTL's job is direct killing of a target cell, and it does it with a precision worth noticing. Its three characteristics are antigen specificity, MHC restriction (class I), and — the detail students forget — the CTL is not injured while killing its target. It kills and moves on to the next one.
| Pathway | How it works |
|---|---|
| Perforin / granzyme | Perforin is a pore-forming protein monomer in CTL granules. It polymerises to form an aqueous channel in the target membrane, so the target can die by osmotic swelling. Granzymes enter through those channels, activate caspases and induce apoptosis |
| Fas–FasL | CTLs express FasL, which binds Fas on the target cell. This activates caspases and causes apoptosis |
Both pathways converge on caspases and apoptosis, and that is not incidental. A virus-infected cell that is simply lysed spills its contents — including intact, infectious virions — into the tissue. Apoptosis instead condenses the cell, fragments its DNA (destroying the viral genome with it), and packages the remains for tidy phagocytosis. The CTL is not just killing the cell; it is destroying the factory and the stock without contaminating the neighbourhood.
- Three characteristics of CTL killing? → Antigen specificity, MHC restriction, the CTL is not injured
- Name the two killing mechanisms. → Perforin/granzyme, and Fas–FasL
- What does perforin do? → Polymerises to form an aqueous channel in the target membrane
- What do granzymes do? → Enter through the channel, activate caspases, induce apoptosis
- Which MHC class restricts CTLs? → Class I
- Why apoptosis rather than lysis? → It destroys the viral genome and avoids spilling infectious virions
Revision layer
The exam map for this unit
| Section | Item | From |
|---|---|---|
| III. MCQ Q11 | αβ T cells participate in adaptive immunity | §7 |
| III. MCQ Q13 | CD40L is expressed on T cells | §6 |
| III. MCQ Q16 | CTLA-4 down-regulates T-cell activation | §6 |
| IV. True/False Q5 | CTL kill via perforin, granzyme and FasL — T | §8 |
The whole unit on one screen
| Question | Answer |
|---|---|
| Three thymic stages? | DN (CD4⁻CD8⁻) → DP (CD4⁺CD8⁺) → SP |
| Positive selection — where, what for? | Cortex, DP stage → MHC restriction |
| Negative selection — where, what for? | Cortico-medullary junction and medulla, SP stage → self-tolerance |
| TCR vs CD3? | TCR binds antigen; CD3 signals via ITAM |
| αβ vs γδ proportion? | 95–99% vs 1–5% |
| CD4 binds? CD8 binds? | β2 domain of MHC II; α3 domain of MHC I |
| Two signals? | TCR–CD3 + peptide–MHC; CD28 + B7 |
| Signal 1 alone gives? | Anergy — peripheral tolerance |
| Accelerators / brakes? | CD28, ICOS / CTLA-4, PD-1 |
| CD40L partner? | CD40 on B cells and APCs |
| Treg markers? | CD4⁺CD25⁺, FoxP3 |
| Th17 cytokines? | IL-17, IL-17F, IL-22 |
| Tfh marker? | CXCR5; secretes IL-21 |
| CTL killing? | Perforin/granzyme, and Fas–FasL — both end in apoptosis |
- Which selection gives MHC restriction, which gives tolerance? → Positive → restriction; negative → tolerance
- What are the two signals for T-cell activation? → TCR–CD3/peptide–MHC, and CD28/B7
- Which molecule is the brake that shares CD28's ligands? → CTLA-4
- Which T cell is adaptive — αβ or γδ? → αβ
- Name the CD4⁺ subsets. → Th1, Th2, Treg, Th17, Tfh
- How does a CTL kill? → Perforin/granzyme and Fas–FasL, both causing apoptosis
- Which co-receptor does HIV use? → CD4