Adaptive Immunity
Usually week 2-4 of an immunology & genetics courseAdaptive immunity is where the exam lives. Innate immunity sets the stage, but the MCQs cluster on what T and B cells actually do: which MHC class presents to which cell, which antibody shows up first versus in memory, and which helper subset drives which disease. Learn the CD4/CD8-to-MHC pairing and the five antibody classes cold — half the question stems resolve on those two tables alone.
This guide is the frame. The exam is written from your lectures— drop this topic's slides on the dashboard to get flashcards and board-style questions from your own course, and quiz them all term (one cumulative exam rewards nothing more than early, repeated self-testing). The immunology & genetics playbook has the weekly loop.
AI-authored study notes, not faculty-reviewed and not medical advice. Verify every mechanism against your course materials and faculty; programs differ in emphasis.
High-Yield Pearl
CD4 × MHC II = 8 and CD8 × MHC I = 8 — pair each with its cargo (exogenous vs endogenous peptide) and half the antigen-presentation questions on the exam answer themselves.
What the exam asks
Mechanisms, patterns, and what happens when each one fails.
- MHC class I vs class II. MHC I is on ALL nucleated cells (not mature RBCs), presents ENDOGENOUS peptides (viral, tumor) processed by the proteasome and loaded via TAP in the ER, and is read by CD8+ T cells — the rule of 8: 1 × 8 = 8. MHC II is restricted to professional antigen-presenting cells (dendritic cells, macrophages, B cells), presents EXOGENOUS peptides processed in acidified endosomes (invariant chain/CLIP swapped out by HLA-DM), and is read by CD4+ T cells — 2 × 4 = 8.
- T-cell activation needs two signals. Signal 1 = the TCR (with CD4 or CD8) engaging peptide-MHC; signal 2 = costimulation, classically CD28 on the T cell binding B7 (CD80/86) on the APC. Signal 1 without signal 2 produces anergy, not activation — the peripheral-tolerance mechanism exam questions love. CTLA-4 and PD-1 are the inhibitory counterparts (the targets of checkpoint-inhibitor drugs).
- CD4 helper subsets. Th1 (driven by IL-12, secretes IFN-γ) activates macrophages — intracellular pathogens, granulomas, type IV hypersensitivity. Th2 (driven by IL-4, secretes IL-4/IL-5/IL-13) drives IgE class switching and eosinophils — helminths and allergy. Th17 (driven by TGF-β + IL-6, secretes IL-17) recruits neutrophils at mucosal surfaces — extracellular bacteria and fungi. Treg (FoxP3+, CD25+, driven by TGF-β) suppresses via IL-10 and TGF-β — peripheral tolerance.
- CD8 cytotoxic T cells. Kill virus-infected and tumor cells presenting antigen on MHC I, using perforin (pore formation) and granzymes (trigger apoptosis via caspases) plus Fas ligand–Fas engagement. They need help getting licensed — dendritic-cell cross-presentation and CD4 help (IL-2) drive full CTL differentiation.
- The five antibody classes. IgM: first antibody made in a primary response, pentamer in serum, best complement activator, the B-cell receptor as a monomer. IgG: most abundant in serum, dominant in the secondary response, the only class crossing the placenta, opsonizes and fixes complement. IgA: dimer with secretory component at mucosal surfaces and in breast milk. IgE: binds mast cells and basophils via FcεRI — type I hypersensitivity and antihelminth defense. IgD: surface receptor on naive B cells, minimal serum role.
- B-cell activation and class switching. T-dependent responses: the B cell presents antigen on MHC II to a Th cell, receives CD40L–CD40 engagement plus cytokines, and in the germinal center undergoes somatic hypermutation (affinity maturation) and class-switch recombination — the isotype changes (constant region) while specificity (variable region) stays. IL-4 switches to IgE, TGF-β to IgA, IFN-γ toward IgG subclasses. T-independent antigens (polysaccharide capsules) give mostly IgM and poor memory — why conjugate vaccines exist.
- Clonal selection and memory. Each lymphocyte carries ONE antigen specificity, generated before antigen exposure by VDJ recombination (RAG1/RAG2). Antigen selects and expands only the matching clones; some progeny persist as memory cells. This is why the secondary response is faster (days shorter lag), larger, higher-affinity, and IgG-dominant — and it is the entire logic of vaccination and booster doses.
- Central vs peripheral tolerance. Central: in the thymus, positive selection (must weakly recognize self-MHC) then negative selection (strong self-reactivity → deletion; AIRE lets the thymus display peripheral self-antigens — AIRE mutation causes APECED/APS-1). B cells are deleted or receptor-edited in the bone marrow. Peripheral: anergy (signal 1 without 2), Treg suppression, and Fas-mediated deletion. Tolerance failure = autoimmunity.
- Key cytokines to pair with sources. IL-2 — T-cell growth factor (autocrine). IL-12 (from APCs) — drives Th1. IFN-γ (Th1, NK) — activates macrophages, upregulates MHC. IL-4 — drives Th2 and IgE switching. IL-5 — eosinophils and IgA. IL-17 — neutrophil recruitment. IL-10 and TGF-β — suppression. IL-6 — acute phase plus Th17 differentiation. Exam stems test cytokine → source → effect as a three-way match.
Worth drilling · 22 concepts
Each with the one fact that identifies it. Your course's slides are the authority; use this to check your list against.
- 1MHC class I — on all nucleated cells; presents endogenous peptide to CD8+ cells
- 2MHC class II — restricted to professional APCs; presents exogenous peptide to CD4+ cells
- 3Dendritic cell — the most potent APC; the only one that activates naive T cells
- 4TAP transporter — pumps proteasome-cut peptides into the ER for class I loading
- 5Invariant chain (CLIP) — blocks the class II groove until HLA-DM swaps in peptide
- 6CD28 — the T cell's costimulatory receptor; binds B7 for signal 2
- 7CTLA-4 — inhibitory receptor that outcompetes CD28 for B7; brake on activation
- 8Th1 cell — IL-12 induced, secretes IFN-γ; activates macrophages against intracellular microbes
- 9Th2 cell — IL-4 induced; drives IgE and eosinophils against helminths and in allergy
- 10Th17 cell — secretes IL-17; recruits neutrophils to mucosal surfaces
- 11Treg — FoxP3+ CD25+ suppressor; keeps peripheral tolerance via IL-10 and TGF-β
- 12Perforin and granzymes — the CTL kill mechanism: pore plus caspase-triggered apoptosis
- 13IgM — pentamer, first responder of the primary response, best complement fixer
- 14IgG — secondary-response workhorse; the only class that crosses the placenta
- 15IgA — dimer with secretory component; guards mucosa and rides in breast milk
- 16IgE — sits on mast cells via FcεRI; type I hypersensitivity and antiparasite defense
- 17CD40 ligand — the T-helper signal a B cell needs to class-switch; missing in hyper-IgM syndrome
- 18Somatic hypermutation — germinal-center point mutations that raise antibody affinity
- 19VDJ recombination — RAG-mediated gene shuffling that builds receptor diversity before antigen
- 20AIRE — thymic factor displaying peripheral self-antigens for negative selection
- 21IL-2 — autocrine T-cell growth factor released on activation
- 22Memory B cell — germinal-center graduate behind the fast, IgG-heavy secondary response
Practice it
Active recall over the drill list — flip and claim, match the pairs, produce the answers cold, fill in the diagram.
Every concept on the drill list, one card at a time — the name up front, the fact that identifies it on the flip. Claim each card honestly and the deck learns what to lead with next time. Progress lives in this browser only.
What everyone misses
The distinctions that lose points on this material, year after year.
- Swapping the MHC pairings. Under time pressure students match CD4 to class I. Use the multiplication rule — CD4 × MHC II = 8, CD8 × MHC I = 8 — and remember the logic: CD8 cells must survey EVERY nucleated cell for viruses, so class I is everywhere; CD4 cells only need to talk to professional APCs.
- Class switching changes specificity. It does not. Class-switch recombination swaps the heavy-chain CONSTANT region (isotype and effector function); the variable region — and therefore the antigen bound — is unchanged. Affinity changes come from somatic hypermutation, a separate germinal-center process.
- IgM titers read as 'more antibody overall'. A high IgM with low IgG means a RECENT primary exposure; high IgG with low IgM means past infection or vaccination. Serology questions are timing questions — answer them off the primary-vs-secondary curve, not off absolute levels.
- Anergy vs deletion. Signal 1 without costimulation makes the T cell ANERGIC — alive but unresponsive (peripheral tolerance). Deletion by apoptosis is negative selection in the thymus (central tolerance) or Fas-mediated in the periphery. MCQs bait you into calling every tolerance mechanism 'deletion.'
- Natural killer cells filed as adaptive. NK cells are INNATE lymphocytes: no TCR, no VDJ rearrangement, no memory in the classic sense. They kill cells that have LOST MHC I ('missing self') — the exact opposite trigger from CD8 T cells, which require MHC I to kill. A stem about a virus downregulating MHC I is asking for NK cells.
- Polysaccharide antigens and infant vaccines. Pure capsular polysaccharide is T-independent — IgM-heavy, weak in children under 2, no real memory. Conjugating it to a protein carrier recruits T-cell help and germinal centers. 'Why is the vaccine conjugated?' is a class-switching question in disguise.
- Th1/Th2 cross-inhibition forgotten. IFN-γ suppresses Th2 and IL-4/IL-10 suppress Th1 — the subsets antagonize each other. Stems about lepromatous vs tuberculoid leprosy or allergy skew are testing which arm won, not which arm exists.
Clinical correlations
Where this topic shows up again — in clinic, on rotations, and on the boards.
- Anaphylaxis and allergy (type I). Prior exposure drives Th2-dependent IgE, which arms mast cells via FcεRI; re-exposure cross-links the IgE → immediate degranulation (histamine, tryptase) → urticaria, bronchospasm, hypotension. Mechanistically this is IgE + Th2 + IL-4; treatment is epinephrine first, and serum tryptase is the lab marker.
- TB skin test and contact dermatitis (type IV). Delayed-type hypersensitivity is Th1 memory cells recognizing antigen, releasing IFN-γ, and activating macrophages over 48–72 hours — no antibody involved. The PPD, poison ivy, and granuloma formation are all the same mechanism, which is also why anti-TNF therapy reactivates latent TB.
- Neonatal passive immunity. Maternal IgG crosses the placenta (the only class that does) and protects the infant for roughly the first 6 months; secretory IgA in breast milk covers the gut. This same IgG transfer is what makes Rh disease possible — anti-D IgG from a sensitized mother attacks fetal red cells, while bulky IgM anti-A/anti-B mostly cannot cross.
- X-linked hyper-IgM syndrome. CD40L deficiency on T cells: B cells never get the class-switch signal, so IgM is high or normal while IgG, IgA, and IgE are low. Boys present with recurrent sinopulmonary infections plus Pneumocystis — a pure demonstration that switching requires T-cell help.
- Asplenia and encapsulated organisms. The spleen's marginal zone handles T-independent responses to polysaccharide capsules and clears opsonized organisms. Splenectomized or sickle-cell patients are at risk from Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria — hence pre-splenectomy vaccination against exactly those.
- Transplant rejection. Hyperacute rejection = preformed antibody against donor antigens, within minutes. Acute rejection = recipient T cells (CD8 direct killing, CD4-driven inflammation) over weeks — the target of calcineurin inhibitors, which block IL-2 production. Chronic rejection = years of low-grade cellular and antibody injury with vascular fibrosis.
- Checkpoint inhibitors and their toxicity. Anti-CTLA-4 and anti-PD-1 antibodies release the brakes on tumor-specific T cells. The predictable price is immune-related adverse events — colitis, thyroiditis, hepatitis, pneumonitis — which are iatrogenic autoimmunity, i.e., peripheral tolerance deliberately switched off.
- Serology in practice. IgM-positive means acute or recent infection (it appears in days and fades in weeks-to-months); IgG-positive alone means past infection or immunization. Hepatitis panels, EBV, and prenatal TORCH interpretation all run on this one primary-vs-secondary rule.