Antigen
The thing that gets recognised
Unit 1 said the immune system's job is to tell self from non-self. This unit is about the non-self β the molecule that actually gets recognised. Everything downstream, every antibody and every T-cell response in the rest of this course, begins with an antigen being seen.
The word is used loosely in conversation and precisely in exams, so fix the precise meaning now. An antigen is not simply 'something foreign'. It is a molecule that can do two specific things: provoke a response, and then be bound by the products of that response. Those two abilities have separate names, they can come apart, and the question that separates them has been on the paper twice.
A molecule that can induce the production of antibodies and/or effector T cells, and can in turn interact specifically with the antibodies and/or effector lymphocytes produced.Section I, 2019
- What two things must a molecule do to be a full antigen? → Induce a response, and bind specifically to that response's products
- Which unit's principle does this build on? → Self versus non-self discrimination (Unit 1)
The two properties of an antigen β β β
Split the definition in half and you have the two properties, which TMU examines by name. Immunogenicity is the ability to provoke β to induce a humoral and/or cell-mediated response, producing antibody and effector lymphocytes. Immunoreactivity, also called antigenicity, is the ability to be bound β to combine specifically with those antibodies or with cell-surface receptors once they exist.
Provoking and being bound are different jobs, and a molecule can be good at the second while being hopeless at the first. Provoking a response needs bulk β enough size and complexity to engage the machinery of two cell types. Being bound needs only a shape that fits. So a very small molecule can be perfectly bindable and yet completely unable to start anything. That gap has a name, and it is the next section.
| Immunogenicity | Immunoreactivity (antigenicity) | |
|---|---|---|
| Ability to | Induce an immune response | Combine specifically with the products of that response |
| Products involved | Antibody and effector lymphocytes are made | Antibody and cell-surface receptors are bound |
| Needs | Size, complexity, foreignness | Only a complementary shape |
| Complete antigen | β Yes | β Yes |
| Hapten | β No | β Yes |
- Define immunogenicity. → The ability to induce a humoral and/or cell-mediated immune response
- Define immunoreactivity/antigenicity. → The ability to combine specifically with antibody or cell-surface receptors
- Which property does a hapten lack? → Immunogenicity β it keeps antigenicity
Complete antigen and hapten β β β
A complete antigen has both properties. A hapten has only the second β it is a small molecule that antibody will bind quite happily, but which cannot by itself provoke the antibody into existence. Give a hapten a carrier, usually a protein, and the combined molecule becomes immunogenic. The immune system then makes antibody that recognises the hapten.
Penicillin is a small molecule, far too small to be immunogenic on its own. But it is chemically reactive: its Ξ²-lactam ring opens and binds covalently to serum proteins, usually albumin. Now it has a carrier, and the conjugate is immunogenic. The immune system responds β and in a susceptible person makes IgE against the penicillin group. On the next exposure, that IgE is waiting, and the result can be anaphylaxis.
This is why the classic hapten example is a drug and not a laboratory curiosity, and why penicillin allergy is a type I hypersensitivity you meet again in Unit 13.
A small-molecular-weight substance which is antigenic but cannot by itself induce a specific immune response β it lacks immunogenicity yet reacts with specific antibody. Coupling to a carrier makes the conjugate immunogenic. A hapten is equivalent to a single epitope and is therefore monovalent.
- A hapten has which property? → Antigenicity (immunoreactivity) only
- What must be added to make a hapten immunogenic? → A carrier, usually a protein
- Give the classic clinical example. → Penicillin, which binds covalently to serum protein
- What is a hapten's valence? → Monovalent β it is equivalent to one epitope
Epitope β where recognition actually happens β β β
An antibody does not recognise a whole bacterium, or even a whole protein. It recognises a small patch on the surface of a molecule β a few residues, no more. That patch is the epitope, also called the antigenic determinant, and it is the true unit of immunological specificity. A large protein carries many of them, which is why one protein can raise many different antibodies at once.
The immunologically active region of an immunogen that binds to antigen-specific membrane receptors on lymphocytes (TCR/BCR) or to secreted antibodies. Epitopes are usually composed of 5 to 15 amino acid residues, or of polysaccharide residues or nucleotides.Section I, 2020
The total number of epitopes on an antigen molecule that can bind antibody. A hapten, being equivalent to a single epitope, is monovalent.
Two ways to classify epitopes
The first cut is structural. A linear (sequential, continuous) epitope is a stretch of consecutive amino acids β and because it is a simple run of sequence, it often lies buried inside the folded molecule. A conformational (non-linear, discontinuous) epitope is made of residues that are far apart in the sequence but brought together by folding, so it exists only while the protein holds its native shape and sits on the surface.
Boil a protein and its conformational epitopes vanish, because the residues that formed them are no longer neighbours β but its linear epitopes survive, and some that were buried are now exposed. This is not trivia: it is why a test that detects an antibody against a conformational epitope must not use denatured antigen, and it is the reason B cells and T cells prefer different epitope types, which is the next distinction.
| T-cell epitope | B-cell epitope | |
|---|---|---|
| Recognised by | TCR | BCR and secreted antibody |
| Must be presented? | Yes β as a peptide bound to an MHC molecule | No β recognised directly on the intact antigen |
| Type | Linear peptide, produced by degradation | Usually conformational |
| Location in the molecule | Anywhere β the antigen is broken up first | On the surface of the native molecule |
- Define epitope. → The immunologically active region of an antigen that binds TCR, BCR or antibody
- How large is a typical epitope? → About 5β15 amino acid residues
- What is antigenic valence? → The number of epitopes on one antigen molecule that can bind antibody
- Which epitope type survives denaturation? → Linear β conformational epitopes are destroyed
- Which epitope type does a T cell see, and how? → A linear peptide, presented on an MHC molecule
Common antigen and cross-reaction β β
Specificity is the great virtue of adaptive immunity, but it is not absolute. If two different antigens happen to carry the same or a similar epitope, antibody raised against one will bind the other. The shared epitope is the common antigen; the unintended binding is the cross-reaction. It can happen between organisms as distant as a bacterium and a human tissue.
The reaction in which antibody induced by one antigen reacts with an unrelated antigen, because the two share an identical epitope or carry epitopes of similar conformation. The shared determinant is called the common antigen.
The cell wall of group A haemolytic streptococcus carries epitopes that resemble those on human cardiac valve glycoprotein, myocardial interstitium and glomerular basement membrane. A child gets a sore throat; the immune system does exactly what it should and makes antibody against the streptococcus; and that antibody then binds the child's own heart valves and kidney. Weeks later come rheumatic fever and acute glomerulonephritis.
Nothing malfunctioned. The specificity was real β it was just not unique. This is molecular mimicry, and it is one of the clearest routes from infection to autoimmune damage.
- Define cross-reaction. → Antibody induced by one antigen reacting with an unrelated antigen
- What makes it possible? → A shared (common) epitope, or two epitopes of similar conformation
- Give the classic clinical pair. → Group A streptococcus β rheumatic fever and post-streptococcal glomerulonephritis
What makes a molecule a good antigen
Immunogenicity is not a property of the molecule alone β it depends on the molecule, on the host meeting it, and on how it is delivered. TMU groups the factors under those three headings, and the grouping itself is worth remembering because it structures the answer.
1 Β· Properties of the antigen
Foreignness comes first and matters most. Following Burnet's clonal selection theory, 'foreign' means a substance that never contacted lymphocytes during embryonic development. That definition has a startling consequence: a molecule of your own can be foreign, if the immune system never met it while the repertoire was being purged. Anything sitting behind the bloodβtestis, bloodβbrain or bloodβeye barrier is in exactly that position.
Lens protein and other eye antigens develop behind the bloodβeye barrier and are never shown to the maturing immune system, so no tolerance to them is ever established. Penetrate one eye with an injury and those sequestered antigens spill out and are seen for the first time β as foreign. The immune response that follows attacks both eyes, and the uninjured one can be blinded. The same logic explains why trauma or vasectomy can raise anti-sperm antibodies.
- Foreignness β different species > different individual of the same species > altered self > unaltered self that never met immune cells
- Chemical nature β proteins and glycoproteins are strong; lipoproteins, polysaccharides and LPS also work; simple molecules do not
- Molecular weight β >100 kD strong, >10 kD adequate, <10 kD poor. Bigger means more epitopes, more complexity, and slower degradation, so the immune cells are stimulated for longer
- Structural complexity β a straight-chain polymer of one amino acid is a poor immunogen even at >100 kD; adding only 2% tyrosine makes it far stronger
- Conformation and accessibility β the epitope has to be reachable, and even ortho- versus meta- versus para- substitution changes recognition
- Physical nature β polymer > monomer; cyclic > linear; particulate > soluble
2 Β· The host
The same molecule is not equally immunogenic in everybody. Genetic background matters most β MHC type determines which peptides can be presented at all, which is why responsiveness to a given antigen runs in families and strains. Age, sex and state of health follow: the very young and very old respond less well, and infection or immunosuppressive drugs blunt the response.
3 Β· How the antigen is given
| Factor | Rule |
|---|---|
| Dose | A moderate dose gives the best response β too little and too much both induce tolerance rather than immunity |
| Timing | Spaced repeat doses beat a single large one |
| Route | Intracutaneous > subcutaneous > intravenous > oral β worth memorising in that order |
| Adjuvant | Non-specifically amplifies the response β see Β§8 |
- What does 'foreign' mean under clonal selection? → Never contacted lymphocytes during embryonic development
- Name the three sequestered-antigen barriers. → Bloodβtestis, bloodβbrain, bloodβeye
- What molecular weight makes a strong immunogen? → Above 100 kD; below 10 kD is poor
- Rank the routes of administration. → Intracutaneous > subcutaneous > intravenous > oral
- Which dose gives the best response? → A moderate one β extremes induce tolerance
Classifying antigens β β
By whether T-cell help is needed
This is the classification that carries the most weight later, because it predicts what kind of response you get. A thymus-dependent (TD) antigen cannot drive antibody production without help from T cells; a thymus-independent (TI) antigen can stimulate B cells directly. The difference is structural β and it explains a real clinical problem.
| TD antigen | TI antigen | |
|---|---|---|
| Needs T-cell help? | Yes | No β stimulates B cells directly |
| Chemical nature | Mainly proteins | Mainly polysaccharides |
| Epitope structure | A few copies of many different determinants | The same determinant repeated many times |
| Response produced | Humoral and cellular | Humoral only |
| Antibody classes | Multiple β IgM, IgG, IgA (class switching occurs) | IgM only |
| Immune memory | Yes | No |
The pneumococcal capsule is a polysaccharide β a classic TI antigen. It gives IgM, no class switching and no memory, and infants under two respond to it especially badly. The fix was to conjugate the polysaccharide to a carrier protein, converting a TI antigen into a TD one. Now T-cell help is recruited, IgG appears, memory forms, and the vaccine protects infants. That is the haptenβcarrier principle from Β§3, used deliberately.
By relationship to the host
| Class | Meaning | Example |
|---|---|---|
| Heterophilic (Forssman) | A common antigen shared across unrelated species β human, animal, plant, microbe | Streptococcal cell wall β cardiac valve and glomerular basement membrane |
| Xenogeneic | From a different species | Microorganisms; therapeutic horse antitoxin serum |
| Allogeneic | From a different individual of the same species | ABO and Rh blood group antigens; MHC/HLA transplantation antigens |
| Autoantigen | The host's own components β altered, or released from a sequestered site | Thyroglobulin, sperm, lens protein |
Therapeutic horse serum given for, say, tetanus is a specific antibody that neutralises the toxin β and simultaneously a xenogeneic antigen that provokes the patient to make antibody against horse protein. That is why repeat courses cause serum sickness, a type III hypersensitivity you meet in Unit 15. The same molecule is the treatment and the next problem.
By where the antigen is synthesised
The last cut matters for Units 10 and 11. Endogenous antigens are generated within the cell and sit in the cytosol β viral proteins made by an infected cell, or tumour proteins. Exogenous antigens are taken up from outside by endocytosis and sit in vesicles. They are handled by different presentation pathways and shown on different MHC classes, which is the whole architecture of antigen presentation.
- TD vs TI β which gives memory? → TD only
- Which antibody class does a TI antigen produce? → IgM only, with no class switching
- What is the structural signature of a TI antigen? → The same epitope repeated many times
- What is a heterophilic antigen? → A common antigen shared between unrelated species
- ABO blood group antigens are which class? → Allogeneic
- Where do endogenous antigens sit? → In the cytosol, made inside the cell
Non-specific stimulators β β
Three things activate lymphocytes without being conventional antigens, and the word that unites them is non-specific. A normal antigen activates the tiny fraction of lymphocytes whose receptors happen to fit it β perhaps one in a hundred thousand. These three bypass that, and activate large fractions of the repertoire at once. That is why they are dangerous, useful, and examinable.
A molecule that polyclonally activates a large fraction of T cells by binding outside the peptide groove β to the MHC class II molecule and the TCR simultaneously. Its features: polyclonal activation Β· no antigen processing required Β· no MHC restriction.
Staphylococcal toxic shock syndrome toxin is a superantigen. Because it clamps MHC II to the TCR from the outside, it does not care what peptide is in the groove, so it activates perhaps 20% of all T cells at once instead of the usual 0.001%. The resulting flood of cytokines β not the bacterium itself β causes the fever, rash, hypotension and multi-organ failure. The damage is done by an immune response that is massive and completely untargeted.
A substance that binds receptors on T or B cells in vitro and stimulates resting lymphocytes to transform into lymphoblasts β larger, with increased DNA synthesis. Activation is polyclonal and non-specific. Common T-cell mitogens: PHA, Con A, PWM.
A substance which, injected before or together with an antigen, non-specifically enhances the immune response or changes its type.
| Adjuvant | Type | Note |
|---|---|---|
| Al(OH)β β alum | Inorganic | The workhorse of human vaccines |
| BCG | Organic | Also used in bladder cancer immunotherapy |
| Poly I:C | Synthetic | Mimics viral double-stranded RNA |
| Freund's β incomplete | Complex | Water-in-oil emulsion |
| Freund's β complete | Complex | Incomplete + killed mycobacteria. Powerful, but too inflammatory for human use β research animals only |
Adjuvants work in three ways, and the list is a tidy short-answer: they change the physical and chemical character of the antigen (typically making a depot that releases it slowly instead of letting it wash away); they improve antigen processing and presentation by macrophages; and they non-specifically stimulate lymphocyte proliferation.
- Define superantigen. → A molecule that polyclonally activates a large fraction of T cells by binding MHC II and TCR outside the peptide groove
- Three features of a superantigen? → Polyclonal activation, no processing needed, no MHC restriction
- Do mitogens act specifically? → No β polyclonally and non-specifically. This is True/False Q1
- Name three T-cell mitogens. → PHA, Con A, PWM
- Define adjuvant. → A substance given with antigen that non-specifically enhances or alters the immune response
- Three mechanisms of adjuvant action? → Alters the antigen's physicochemical form; improves processing and presentation; stimulates lymphocyte proliferation
Revision layer
The exam map for this unit
| Section | Item | From |
|---|---|---|
| I. Definitions | Antigen β set 2019 | Β§1 |
| I. Definitions | Epitope β set 2020 | Β§4 |
| III. MCQ Q2 | Haptens have antigenicity but no immunogenicity | Β§3 |
| IV. True/False Q1 | Mitogens activate specifically β F | Β§8 |
Antigen and Epitope were set in consecutive years, one each β just as Immunoglobulin and Antibody were, and APCs and MHC. The department appears to rotate within a pair rather than introduce new terms. Prepare both halves of each pair and you are covered whichever way it falls.
The whole unit in one table
| Concept | One-line answer |
|---|---|
| Antigen | Induces antibody/effector T cells and binds them specifically |
| Immunogenicity | Ability to induce a response |
| Antigenicity | Ability to be bound by the response's products |
| Hapten | Antigenic but not immunogenic; needs a carrier; monovalent |
| Epitope | The 5β15 residue patch that TCR/BCR/antibody actually binds |
| Valence | Number of bindable epitopes per antigen molecule |
| Linear epitope | Consecutive residues; survives denaturation; often buried |
| Conformational epitope | Folded-together residues; surface; destroyed by denaturation |
| Cross-reaction | Antibody to one antigen binds an unrelated one sharing an epitope |
| TD antigen | Protein; needs T help; IgMβIgG switching; memory |
| TI antigen | Polysaccharide; repeated epitope; IgM only; no memory |
| Superantigen | Binds MHC II + TCR outside the groove; polyclonal; no MHC restriction |
| Mitogen | Polyclonal, non-specific lymphocyte activation in vitro |
| Adjuvant | Non-specifically enhances or redirects the response to a co-injected antigen |
- The two properties of an antigen? → Immunogenicity and immunoreactivity (antigenicity)
- Hapten + what = immunogenic? → A carrier protein
- Epitope size? → 5β15 amino acid residues
- Which epitope type do B cells prefer? → Conformational, on the native surface
- Streptococcus β heart valve is an example of? → Cross-reaction via a common (heterophilic) antigen
- Best route of antigen administration? → Intracutaneous
- TI antigens give which antibody, and how much memory? → IgM only; no memory
- Are mitogens specific? → No β that is True/False Q1, and the answer is False