Adaptive Antiviral Immunity — Questions

• What is recognised. A viral peptide displayed on major histocompatibility complex (MHC) class II by a professional antigen-presenting cell (dendritic cell, macrophage, or B cell).
• The molecular interaction. The T cell receptor (TCR), its α and β variable regions from V(D)J recombination, binds the peptide–MHC class II complex; the CD4 co-receptor binds the conserved β2 domain and recruits the kinase Lck; the CD28 co-stimulator binds CD80 or CD86 (B7) to give the second signal (TCR engagement without CD28 induces anergy).
• Restriction. Recognition is MHC class II-restricted, not class I.
• The epitope. A linear peptide of 13 to 18 amino acids that can overhang the open-ended class II groove, generated from extracellular antigen through the endocytic pathway (cathepsin cleavage, invariant-chain release, HLA-DM-mediated loading).
• HLA dependence. Only peptides fitting an individual’s HLA class II alleles (HLA-DR, HLA-DP, HLA-DQ) are presented. CD4+ cells then license both the cytotoxic and the B cell programmes, which is why HIV-1 depletion of CD4+ T cells disables antibody affinity maturation, CD8+ priming, and Th1 macrophage activation together.

• What is recognised. A viral peptide displayed on major histocompatibility complex (MHC) class I of an infected nucleated cell.
• The molecular interaction. The T cell receptor (TCR), its α and β variable regions generated by V(D)J recombination, binds the peptide–MHC class I complex; the CD8 co-receptor binds the conserved α3 domain and recruits the kinase Lck; naïve priming also needs CD28 to engage CD80 or CD86 (B7) on a mature dendritic cell, though effector killing needs no co-stimulation.
• Restriction. Recognition is MHC class I-restricted, and because every nucleated cell expresses class I, every cell is open to surveillance.
• The epitope. A linear peptide of 8 to 11 amino acids held in the closed-ended class I groove, derived from cytoplasmic viral proteins cleaved by the proteasome and imported to the endoplasmic reticulum by the TAP transporter.
• HLA dependence. Only peptides whose anchor residues fit an individual’s HLA class I alleles (HLA-A, HLA-B, HLA-C) are presented, so responses vary with HLA type; protective alleles such as HLA-B57 present conserved HIV-1 Gag epitopes that are hard for the virus to escape. Epitopes favour conserved internal proteins, making CD8+ responses more cross-reactive between strains than antibody.

• Definition. Affinity maturation is the progressive rise in antibody antigen-binding affinity during a response, so antibody made late or on re-exposure binds far more tightly than at first encounter.
• Where and when. In the germinal centres of secondary lymphoid organs (lymph nodes, spleen, Peyer’s patches), over the one to three weeks after a B cell is activated.
• Somatic hypermutation. Activation-induced cytidine deaminase (AID) introduces point mutations in the rearranged immunoglobulin variable (V) genes at about a million times the background rate, concentrated in the antigen-contacting complementarity-determining regions.
• Clonal selection. Mutated B cells compete for antigen held on follicular dendritic cells; higher-affinity receptors capture more antigen and present more peptide on MHC class II.
• Tfh help and cycling. T follicular helper cells deliver limiting survival signals (CD40 ligand, IL-21) to the best binders as B cells cycle between the dark zone (mutation) and light zone (selection). The outputs are long-lived bone-marrow plasma cells and affinity-matured memory B cells. Failure of the process (AID deficiency, X-linked CD40 ligand deficiency) causes hyper-IgM syndrome.

• Principle. Antibody made early in infection binds weakly; after affinity maturation in germinal centres it binds strongly, so IgG avidity separates recent from past primary infection.
• Affinity versus avidity. Affinity is the strength of a single antibody–antigen bond; avidity is the total binding of a complex, set by the individual affinities and the valency.
• Method. Two parallel ELISAs on the same sample, one with a brief chaotropic agent step (urea, diethylamine, ammonium thiocyanate) after binding that strips low-avidity bonds but not high-avidity ones.
• Avidity index. AI = (OD with chaotrope) ÷ (OD without chaotrope) × 100. Low (below ~30 to 40%) indicates recent primary infection in the past two to four months; high (above ~60%) indicates infection at least three to six months earlier.
• Clinical use. Greatest value is in pregnancy screening, where the timing of maternal infection sets fetal risk: high early-pregnancy avidity for toxoplasma, cytomegalovirus, or rubella excludes recent primary infection, while low avidity raises concern. It also clarifies persistent or non-specific IgM by checking whether IgG has matured.

• Capture step. The plate is coated with anti-human IgM antibody, which captures all IgM from the patient’s serum (specific and non-specific); other classes (IgG, IgA) are washed away.
• Detection step. Viral antigen is added and binds only the IgM of matching specificity; bound antigen is then detected with a labelled anti-viral reagent, giving a signal proportional to virus-specific IgM.
• Why the capture format is preferred. It solves two problems of a direct IgM ELISA: rheumatoid factor (itself an anti-IgG IgM) cannot cause false positives because no IgG remains in the captured fraction, and high-titre virus-specific IgG cannot outcompete IgM for antigen because it has been washed off.
• Clinical use. The test of choice for recent or current infection, since IgM appears in the first one to two weeks and decays over months. Applications include cytomegalovirus, rubella, toxoplasma, and parvovirus B19 in pregnancy, newborn screening for intrauterine infection (IgM does not cross the placenta), and anti-HBc IgM in acute hepatitis B. Combining it with IgG avidity improves discrimination of persistent IgM.

Adaptive immunity removes virus-infected cells through several non-redundant mechanisms.

Cytotoxic T lymphocyte (CTL) killing. CD8+ CTLs recognise viral peptides on MHC class I of infected cells. After priming by a mature dendritic cell (with CD4+ help via CD40), effector CTLs kill by perforin and granzyme (synapse formation, pore-forming perforin, granzyme serine proteases driving caspase-mediated apoptosis) and by Fas ligand (FasL)–Fas (CD95), which assembles the death-inducing signalling complex and activates caspase-8. One CTL kills several targets serially.

Antibody-driven killing. Three routes. Neutralisation: IgG and secretory IgA bind viral glycoproteins and block receptor binding, uncoating, or fusion, the dominant means of preventing new infection. Antibody-dependent cellular cytotoxicity (ADCC): IgG on the infected-cell surface is recognised by natural killer (NK) cell CD16 (FcγRIII), triggering perforin and granzyme release. Complement: IgG and IgM activate the classical pathway via C1q, depositing C3b and assembling the membrane attack complex (MAC, C5b–C9).

Macrophage activation by CD4+ Th1 cells. Th1 cells secrete IFN-γ, which activates macrophages, upregulates MHC class I on bystander cells for CTL recognition, and recruits further effectors.

Intracellular antibody-mediated immunity (TRIM21). When antibody-coated virions reach the cytoplasm (a non-enveloped virus crossing the endosomal membrane), the intracellular Fc receptor TRIM21 ubiquitinates the antibody–virus complex for proteasomal degradation and activates innate sensors. It acts against adenoviruses and other antibody-coated non-enveloped viruses.

Clinical anchor. Patients with B cell defects (X-linked agammaglobulinaemia) clear most acute infections by CTL killing but cannot prevent re-infection and are uniquely vulnerable to enteroviruses; patients with T cell defects (SCID, DiGeorge) fail to mount CTL responses and die from disseminated herpesvirus disease and giant-cell measles pneumonitis. Each arm controls a distinct set of viral infections.

T cells see short peptides displayed by major histocompatibility complex (MHC) molecules. The pathway a peptide takes determines which MHC class presents it and which T cell subset inspects it.

MHC class I (endogenous) pathway. Every nucleated cell samples its own cytoplasmic proteins, including viral ones. Ubiquitin-tagged proteins are degraded by the proteasome into 8 to 11-residue peptides (interferon induces the immunoproteasome). Peptides enter the endoplasmic reticulum via the transporter associated with antigen processing (TAP), where calnexin, calreticulin, ERp57, and tapasin load them onto nascent MHC class I for CD8+ inspection. The groove is closed at both ends.

MHC class II (exogenous) pathway. Restricted to professional antigen-presenting cells (APCs): dendritic cells, macrophages, B cells. Extracellular antigen is taken up and digested by cathepsins into longer peptides (13 to 18 residues). The invariant chain blocks the groove until cathepsins trim it to CLIP, and HLA-DM swaps CLIP for antigenic peptide, presented to CD4+ helper T cells. The groove is open-ended.

Cross-presentation. The specialised cDC1 diverts extracellular viral antigen into the class I pathway, priming CD8+ T cells against viruses that do not infect dendritic cells directly, essential for hepatotropic and other tissue-restricted viruses primed in the draining lymph node.

CD1 and class Ib molecules. CD1 presents lipid and glycolipid antigens to unconventional T cells; class Ib molecules (HLA-E, HLA-F, HLA-G) present restricted peptides and regulate NK cells and feto-maternal tolerance.

Clinical anchor. TAP1 or TAP2 deficiency abolishes class I loading, giving chronic necrotising granulomatous skin lesions; severe viral disease is less profound than expected because cross-presentation and NK defence partly compensate.

(a) The B cell receptor (BCR). A membrane-anchored immunoglobulin (membrane IgM with IgD on a naïve cell), whose variable regions match the antibody the cell will secrete. Unlike the T cell receptor (TCR), which sees a processed peptide on MHC, the BCR binds native antigen directly; antigen cross-links it, triggering internalisation and presentation on MHC class II for T helper recognition.

(b) B cell epitopes. The surface an antibody binds. They are usually conformational (residues distant in sequence but folded together) and require native conformation, so denaturation often destroys antibody binding (relevant in Western blot) without affecting T cell recognition. Only some epitopes are exposed on the intact virion and so relevant to neutralising antibody.

(c) T-dependent and T-independent activation. Activation needs two signals: signal 1 is BCR cross-linking; signal 2 is either CD4+ help (T-dependent) or strong innate or repetitive-antigen co-stimulation (T-independent). T-dependent activation dominates for all protein (viral) antigens: the B cell presents peptide on class II to T follicular helper (Tfh) cells in a germinal centre, which deliver CD40 ligand and cytokines, driving hypermutation, affinity maturation, class switching, and memory. T-independent activation serves repetitive polymers (polysaccharides, capsids), producing mainly IgM with poor memory, which is why conjugate vaccines that add a protein carrier work where plain polysaccharide vaccines fail in infants.

(d) Class switching. The heavy-chain constant region sets isotype; switching substitutes IgM for IgG, IgA, or IgE while preserving specificity. It is a DNA recombination catalysed by activation-induced cytidine deaminase (AID), directed by cytokine context: IFN-γ to IgG1/IgG3, IL-4 to IgE, and TGF-β and the mucosal milieu to IgA. Defective switching causes hyper-IgM syndrome (X-linked CD40 ligand deficiency, or AID deficiency): only IgM, with severe Pneumocystis, cryptosporidium, and disseminated CMV.

A complete answer covers what defines a bNAb, the conserved epitope classes on Env, why the antibodies are so hard to elicit, and their therapeutic, preventive, and vaccine implications.

Definition. A broadly neutralising HIV antibody neutralises a wide cross-section of HIV-1 isolates across genetic clades, conventionally at least 50% of a pseudovirus panel. They arise spontaneously in around 10 to 20% of infected individuals, typically after two or more years of chronic viraemia.

Major epitope classes. The Env trimer of gp120 and gp41 carries six conserved targets: the CD4 binding site (VRC01, 3BNC117, N6); the V1V2 apex (PG9, PG16, CAP256-VRC26); the V3 glycan supersite at the N332 glycan (PGT121, 10-1074); the MPER of gp41 (2F5, 10E8, often autoreactive); the fusion peptide (VRC34); and the gp120–gp41 interface (PGT151, 8ANC195).

Why hard to elicit. Env defends through a glycan shield (about half its surface mass is host glycan), conformational masking of functional epitopes, hypervariable decoy loops, and a need for extreme somatic hypermutation (20 to 40% divergence from germline versus about 5% typically), requiring years of germinal-centre cycling.

Therapeutic and preventive use. Passive infusion (VRC01, 3BNC117, 10-1074) lowers viraemia and is explored for functional cure. The AMP trials showed VRC01 protected against neutralisation-sensitive viruses, proof of concept for prevention; triple-bNAb cocktails and Fc LS mutations for twice-yearly dosing are in development.

Vaccine implications. Inducing bNAbs is the central HIV vaccine challenge. Germline-targeting immunogens (eOD-GT8) activate VRC01-class precursors, and sequential immunisation then drives maturation. Earlier approaches reached at best 31% efficacy (RV144).

A complete answer covers the principle of passive immunisation, how monoclonal antibodies are made and engineered, the licensed preventive and therapeutic indications, the mechanisms of protection, and the limitations that constrain use.

Principle. A monoclonal antibody (mAb) is a population of antibodies of a single defined specificity. Given therapeutically, it provides immediate, defined-specificity humoral protection without depending on the recipient’s own B cell response: passive immunisation, the modern descendant of serum therapy.

Production and engineering. The original method (Köhler and Milstein, 1975) fuses an immunised mouse B cell with a myeloma cell to make an immortal hybridoma. Modern methods add phage display, single-B-cell sequencing from convalescent donors, and transgenic mice with human immunoglobulin loci. Engineering reduces immunogenicity along a ladder from murine (-omab) to chimeric (-ximab), humanised (-zumab), and fully human (-umab). The Fc region can be engineered to extend half-life (YTE and LS mutations enhance neonatal Fc receptor binding), to enhance ADCC (afucosylation), or to abolish effector function.

Prevention. Palivizumab (anti-RSV F glycoprotein) gives monthly prophylaxis in high-risk infants; nirsevimab adds an extended half-life so a single dose covers a season, with trial efficacy around 80% against medically attended RSV lower respiratory tract infection. Tixagevimab–cilgavimab was licensed for pre-exposure prophylaxis in profoundly immunocompromised adults until the Omicron variants escaped both antibodies.

Treatment. SARS-CoV-2 cocktails (casirivimab–imdevimab, sotrovimab, bebtelovimab) were each withdrawn as variants escaped their epitopes, the textbook demonstration of variant escape. Ebolavirus: Inmazeb (a three-mAb cocktail) and ansuvimab (single mAb) are licensed for Zaire ebolavirus, validated in the PALM trial. HIV-1: ibalizumab, an anti-CD4 entry inhibitor, treats multidrug-resistant infection, and broadly neutralising antibodies (VRC01, 3BNC117) are in trials.

Mechanisms. Neutralisation (blocking receptor binding, fusion, or uncoating) is dominant; ADCC via NK cell CD16 and complement activation also contribute; ibalizumab acts by receptor blockade.

Limitations. Variant escape (mitigated by non-overlapping-epitope cocktails), high cost of mammalian-cell production, parenteral administration, and finite half-life (extended by Fc engineering). mAbs are most valuable when a vaccine would be too slow (post-exposure rabies, ebolavirus), the recipient cannot respond (immunocompromised, infants), or the target is highly conserved (RSV F glycoprotein), so they complement rather than replace small-molecule antivirals and vaccines.

A dendritic cell not infected by the virus has no viral protein in its cytoplasm to load onto MHC class I, so cross-presentation is what allows a CD8+ response against viruses that spare dendritic cells (hepatitis B, which replicates only in hepatocytes, is the classic example). The subset that performs it is the conventional dendritic cell type 1 (cDC1), marked in humans by CD141 (BDCA3), XCR1, and CLEC9A, and dependent on the transcription factors IRF8 and BATF3. It takes up antigen from infected cells and diverts it onto MHC class I by a cytosolic route (endosome to proteasome to TAP) or a vacuolar route (endosomal cathepsin loading onto recycling class I).

Plasmacytoid dendritic cells are the principal type I interferon producers, not cross-presenters; cDC2 and Langerhans cells specialise in MHC class II presentation to CD4+ cells; monocyte-derived dendritic cells do not specialise in cross-presentation.

IRF8 deficiency abolishes the cDC1 lineage and produces severe susceptibility to mycobacteria and viruses. cDC1-targeting vaccine strategies (anti-CLEC9A antigen fusions) aim to boost CD8+ responses.

Original antigenic sin is the host phenomenon in which memory of the first viral encounter dominates the response to later, related variants. Memory B cells recognising shared (cross-reactive) epitopes outcompete naïve B cells for antigen and Tfh help, so the boosted antibody is biased toward the priming strain even when the dominant neutralising epitopes have changed. If the new variant’s protective epitopes are not shared, the response is serologically impressive but clinically ineffective.

The distractors name different concepts. Antigenic drift is the gradual point-mutation of surface glycoproteins (a property of the virus), and antigenic shift the abrupt reassortment of a whole gene segment; both describe the virus, not the host response. Antibody-dependent enhancement is the downstream harm when cross-reactive, poorly neutralising antibody enhances rather than blocks infection, the textbook mechanism of severe secondary dengue. Heterologous immunity is cross-protection between unrelated viruses through cross-reactive memory T cells.

Universal influenza vaccine strategies try to circumvent the effect by directing the response toward conserved epitopes (the haemagglutinin stalk, the matrix protein) shared across strains.

T follicular helper (Tfh) cells provide the cognate help a B cell needs for a productive germinal centre response. Positioned in the B cell follicle by the chemokine receptor CXCR5 and the master transcription factor Bcl-6, their dominant cytokine output is IL-21, which supports somatic hypermutation, affinity maturation, and class switching.

Th1 (IFN-γ, T-bet) activates macrophages and supports CD8+ cytotoxic responses; Th2 (IL-4, GATA-3) drives helminth and allergic responses; Th17 (IL-17 and IL-22, RORγt) defends against extracellular bacteria and fungi; regulatory T cells (IL-10 and TGF-β, FoxP3) restrain responses and maintain self-tolerance. None supplies the germinal centre B cell help that defines Tfh.

Defects in the transcription factor STAT3 abolish Th17 development and produce hyper-IgE (Job) syndrome, with recurrent Staphylococcus aureus skin and lung infection. FoxP3 mutations cause IPEX (immunodysregulation polyendocrinopathy enteropathy X-linked), where loss of regulatory T cells drives severe autoimmunity in infancy.

The MHC class II (exogenous) pathway samples proteins taken up from outside the cell, and is restricted to professional antigen-presenting cells: dendritic cells, macrophages, and B cells. Antigen is endocytosed and cleaved by cathepsins in acidifying endosomes. Nascent class II carries the invariant chain (Ii), which blocks the groove; in the late endosomal compartment cathepsins trim it to a fragment called CLIP (class II-associated invariant chain peptide), and the chaperone HLA-DM swaps CLIP for an antigenic peptide before the complex reaches the surface.

Option A is the class I (endogenous) pathway; cross-presentation (D) is real but loads exogenous antigen onto class I, not class II; options B and E are fictitious.

Bare lymphocyte syndrome type II (mutations in CIITA, RFX5, RFXANK, or RFXAP) abolishes MHC class II expression: affected patients cannot prime CD4+ responses and develop a combined immunodeficiency that is fatal without haematopoietic stem cell transplantation.

The MHC class I (endogenous) pathway samples cytoplasmic proteins, including viral proteins made by an infected cell. Ubiquitin-tagged proteins are degraded by the proteasome into 8 to 11-residue peptides (interferon induces the immunoproteasome for class I-optimised peptides); the transporter associated with antigen processing (TAP) moves them into the endoplasmic reticulum, where calnexin, calreticulin, ERp57, and the adaptor tapasin load them onto nascent MHC class I for trafficking to the surface and CD8+ inspection.

Option A is the class II pathway; autophagy (B) delivers cytoplasmic material to lysosomes, not to class I; options C and D are fictitious.

TAP deficiency abolishes class I peptide loading, giving low surface MHC class I, chronic necrotising granulomatous skin lesions, and recurrent bacterial sinopulmonary infection. Viral disease is milder than expected because cross-presentation and NK cell missing-self recognition partly compensate.

Tissue-resident memory T cells (TRM), marked by CD103 and CD69, sit permanently in the tissue and can contain a local re-infection within minutes, before circulating cells could be recruited from the blood. Most viral infections enter through a mucosal or cutaneous surface, so this positioning matters: skin TRM suppress varicella-zoster reactivation, lung TRM give heterosubtypic influenza protection, and genital-tract TRM control herpes simplex virus type 2 recurrences.

Central and effector memory cells recirculate or patrol rather than residing at a fixed site; long-lived plasma cells are a B lineage; plasmacytoid dendritic cells are innate.

Tissue TGF-β drives CD103 expression and TRM commitment, and IL-15 maintains them long-term. Because parenteral vaccines induce mainly central memory and serum antibody, interest in intranasal and mucosal vaccines is driven largely by the need to seed TRM at the portal of entry.

Inactivated whole-virus vaccines present killed antigen that never reaches the cytoplasm, so it is loaded onto MHC class II but poorly onto MHC class I. The intact surface epitopes make excellent neutralising antibody, but CD8+ cytotoxic T cell priming is weak, which is why these vaccines are usually adjuvanted (alum, MF59, AS04) and given as several doses. Inactivated polio, hepatitis A, rabies, and inactivated influenza are examples.

The other platforms generate endogenous antigen inside host cells and so prime both antibody and CD8+ responses: live-attenuated (MMR, oral polio), viral-vector (adenovirus-vectored COVID-19, rVSV Ebola), and mRNA (BNT162b2, mRNA-1273). DNA vaccines work on the same endogenous-antigen principle but need efficient delivery and have no widely licensed human product.

Platform choice follows the protective mechanism the target virus needs: antibody-dominant threats (rabies, hepatitis B, HPV) suit subunit or inactivated vaccines, while viruses that need CD8+ control favour live-attenuated, viral-vector, or mRNA platforms. Live-attenuated vaccines are contraindicated in profound immunocompromise.

• Core principle. A vaccine exposes the immune system to a non-pathogenic facsimile of a virus, priming the same memory pools as natural infection without causing disease, so re-exposure draws a fast memory response rather than a slow naïve one.
• Memory generated. Memory B cells, long-lived bone-marrow plasma cells, and memory CD4+ and CD8+ T cells (central, effector, and tissue-resident).
• Levels of protection. Sterilising immunity prevents infection (rare; HPV vaccines come close); disease prevention allows infection but blocks illness; severity reduction blocks only severe outcomes (as with drifting influenza and SARS-CoV-2).
• Mucosal versus systemic. Parenteral vaccines give serum IgG but little mucosal cover; live mucosal vaccines (oral polio, intranasal influenza) induce secretory IgA at the portal of entry and block transmission better.
• Herd immunity. Above a coverage threshold the effective reproductive number falls below one and transmission collapses; the threshold tracks R₀ (around 95% for measles), and it underpins eradication strategy. Only smallpox has been eradicated by vaccine.

A genome of around 20,000 genes must generate a receptor repertoire of more than 10¹² specificities. The solution is somatic recombination of immunoglobulin and T cell receptor gene segments, discovered by Susumu Tonegawa (Nobel Prize 1987).

• V(D)J recombination. Each variable region is assembled from a few segments drawn from large V (variable), D (diversity), and J (joining) pools, joined during lymphocyte development by RAG1 and RAG2 (recombination activating genes).
• Combinatorial diversity. Random selection of one V, D, and J segment; for the immunoglobulin heavy chain alone this gives thousands of combinations before any further diversity.
• Junctional diversity. Imprecise joining, with terminal deoxynucleotidyl transferase (TdT) adding random nucleotides and nucleases trimming others, at the antigen-contacting junction.
• Chain pairing and exclusion. Any heavy chain may pair with any light chain (multiplying diversity), while allelic exclusion keeps each cell to one specificity.
• Somatic hypermutation. After antigen encounter, AID (activation-induced cytidine deaminase) mutates rearranged V regions in germinal centres, the basis of affinity maturation.

Children with RAG1 or RAG2 mutations cannot complete V(D)J recombination and develop severe combined immunodeficiency (SCID), lacking both T and B cells; hypomorphic mutations cause Omenn syndrome.

• Definition. Cross priming (cross-presentation) is the loading of extracellularly acquired viral antigen onto MHC class I by dendritic cells, priming CD8+ cytotoxic T cells.
• The problem it solves. The class I pathway normally samples only the presenting cell’s own cytoplasmic proteins, so a virus that does not infect dendritic cells would otherwise leave them nothing to load and no way to prime a CD8+ response. Many important viruses fit this pattern: hepatitis B replicates only in hepatocytes, and rabies in muscle and neurons.
• The cell responsible. Almost exclusively cDC1 (conventional dendritic cell type 1), marked by CD141 (BDCA3), XCR1, and CLEC9A and dependent on IRF8; a minority of dendritic cells but essential for most antiviral CD8+ responses.
• Mechanism. Two routes: the cytosolic route, where antigen escapes the endosome to the cytoplasm and enters the proteasome–TAP–class I pathway, and the vacuolar route, where cathepsins process it in the endosome for loading onto recycling class I.
• Importance. It primes hepatitis B-specific CD8+ cells in the draining lymph node despite strict hepatotropism, and underlies the ability of protein-subunit vaccines to elicit cytotoxic responses.

A naïve response is the first encounter with an antigen; a memory response is every subsequent one. They differ in kinetics, magnitude, antibody quality, and effector readiness.

• Cellular substrate of memory. Long-lived bone-marrow plasma cells (years of antibody), memory B cells (rapid plasma-cell differentiation on re-exposure), and the three memory T cell subsets: central (TCM), effector (TEM), and tissue-resident (TRM).
• Why it matters. A naïve host lets a new virus replicate unopposed for days; a memory host shuts the same virus down at the inoculum, often without symptoms. Vaccines exploit this by laying down memory in advance.
• Clinical use. The IgM-to-IgG transition underlies serological staging: IgM with low-avidity IgG indicates recent primary infection, while high-avidity IgG without IgM indicates past infection.

Active immunity is generated by the host’s own immune system in response to antigen; passive immunity is conferred by transferring pre-formed antibody from another source.

• Natural active. Wild-type infection leaves lasting memory, protecting for decades against viruses without antigenic variation.
• Natural passive. Chiefly transplacental maternal IgG, protecting the newborn for around six to twelve months (and the reason live measles vaccine is deferred to 9 to 12 months), plus secretory IgA in colostrum.
• Combined active-plus-passive prophylaxis covers the gap when the inoculum has already arrived: rabies (immunoglobulin around the wound plus vaccine series), and the newborn of an HBsAg-positive mother (HBIG within 12 hours plus hepatitis B vaccine). Passive immunity buys time; active immunity provides durable protection.