Every cell carries molecular identity tags called antigens on its surface. The immune system reads these tags, lets the body's own cells through, and mounts a layered response against anything that isn't self. The response runs at three speeds: physical and chemical barriers, the non-specific cellular response, and the specific response that remembers.
Antigens on the cell-surface membrane let the immune system distinguish self from non-self.
Antigens are molecules — typically glycoproteins or glycolipids — embedded in the outer leaflet of the cell-surface membrane. Their specific shape determines whether a cell reads as self (belonging to the organism) or non-self (foreign). Four categories of cell stimulate an immune response, and these are different from the cells that take part in delivering the response.
Write
cell-surface membrane, notsurfaceorcell membrane. AQA's full term carries the location-specific meaning. The shorter forms are not credited.
Pathogens — bacteria, viruses, fungi, protoctista carrying non-self antigens. Cells from another organism of the same species — transplanted tissue carrying the donor's antigens. Cancer cells — abnormal antigens not present on healthy versions of the same cell type. Cells infected with viruses — display fragments of viral antigen on their surface, marking them as targets.
Cells that
stimulatean immune response are not the same as cellsinvolved inthe response. Pathogens, infected cells, cancer cells, and foreign cells stimulate. B cells, T cells, and phagocytes are involved in delivering the response. Misreading the question's verb costs more than half the marks.
Skin — multiple layers of keratinised epithelium form a continuous, largely impermeable covering. Mucus and ciliated epithelium — the respiratory tract traps inhaled particles in mucus, and cilia beat the mucus toward the throat to be swallowed. Stomach acid — low pH denatures the proteins of most ingested bacteria. Natural flora — established microorganisms on skin and in the gut compete with pathogens for nutrients and space.
The non-specific response engulfs pathogens and presents their antigens to the specific system.
When a pathogen breaches the physical barriers, the non-specific response activates immediately and treats any foreign material the same way. Inflammation, driven by histamine, brings white blood cells and plasma proteins to the infected tissue. Lysozymes in tears and mucus degrade bacterial cell walls. The cellular core of the response is phagocytosis.
- A phagocyte (typically a neutrophil or macrophage) encounters the pathogen, attracted to the site by chemicals released from damaged cells.
- The phagocyte engulfs the pathogen by extending pseudopodia around it, enclosing it in a membrane-bound vesicle called a phagosome.
- The phagosome fuses with a lysosome, exposing the pathogen to the hydrolytic enzymes inside the combined phagolysosome.
- The hydrolytic enzymes digest the pathogen, breaking it down into its component molecules.
- Antigens from the pathogen are displayed on the phagocyte's cell-surface membrane. The phagocyte has now become an antigen-presenting cell, ready to activate the specific immune system.
The
phagosomefuses with thelysosome— not the pathogen, not the virus. The pathogen is already enclosed inside the phagosome before fusion occurs.
Antigen display on the cell-surface membrane is a distinct mark point. Most students stop the chain at digestion and never describe the presentation step. It is consistently the most-dropped step in extended-writing answers.
Antigen presentation is the bridge between non-specific and specific immunity. T helper cells recognise the displayed antigens and trigger the specific response. The full mark-scheme chain continues from step 5 into B cell activation, clonal expansion, and antibody secretion.
Pitfall — The phagocytosis-to-antibody chain has a fixed order
Name every step in sequence; each one earns a separate mark.
AQA's extended-writing answer runs continuously through the full chain: phagocyte engulfs pathogen, phagosome fuses with lysosome, enzymes digest, antigens displayed on the cell-surface membrane, T helper cell binds the displayed antigen, T helper stimulates the specific B cell, B cell divides by mitosis (clonal expansion), plasma cells secrete the specific antibody.
The two most consistently dropped steps are antigen display (step 5 of phagocytosis) and clonal expansion (the mitotic division step). 2018 P1 Q07.5 had a 7.3% full-mark rate on this chain.
T and B lymphocytes deliver the specific response; antibodies bind antigens and cause agglutination or neutralisation.
Specific immunity is antigen-specific and generates memory. Two lymphocyte classes carry it. T lymphocytes mature in the thymus and deliver the cell-mediated response through direct cell-to-cell interaction. B lymphocytes mature in the bone marrow and deliver the humoral response through secreted antibodies. Each class differentiates into functional subtypes.
T helper cells recognise antigens displayed on antigen-presenting cells and secrete chemicals that activate B cells and T killer cells; they are the coordinators of the entire specific response. T killer cells (cytotoxic) destroy cells displaying foreign antigens — virus-infected cells, transplanted cells, cancer cells — by releasing cytotoxic proteins. Memory T cells persist for decades and enable rapid secondary responses.
Use the full term
T helper cell, notT cellalone. The qualifier identifies which subtype is acting and is required for the activation mark on B cell stimulation questions.
When a B cell's surface antibody binds its complementary antigen, a T helper cell stimulates it. The B cell then divides rapidly by mitosis in a process called clonal expansion, producing two populations: plasma cells, which secrete the specific antibody, and memory B cells, which persist for rapid secondary response.
Plasma cellssecrete antibodies — not B cells, and never T cells.B cell secretes antibodiesloses the activation mark;T cell produces antibodiesis an outright reject.
A Y-shaped glycoprotein built from four polypeptide chains — two heavy and two light — held together by disulfide bonds. The two tips of the Y are the variable regions, each carrying one antigen-binding site whose shape is complementary to one specific antigen. Two binding sites per antibody make it bivalent: one antibody can bridge two antigens.
When an antibody binds its antigen, an antigen-antibody complex forms — the credited intermediate between binding and destruction. From the complex, three mechanisms operate. Agglutination cross-links pathogens into clumps that phagocytes engulf in bulk. Marking for phagocytosis coats individual pathogens so phagocytes recognise and engulf them more readily. Neutralisation applies to toxins specifically: the antibody binds the toxin and blocks its activity. The toxin is not destroyed; its harmful effect is prevented.
Primary and secondary responses compared.
| Response | Speed | Antibody concentration peak | Cause |
|---|---|---|---|
| Primary | Slow (days) | Lower | B cells must undergo clonal expansion from scratch |
| Secondary | Fast (hours) | Much higher | Memory cells already present, rapidly activated |
Immunity is active or passive, natural or artificial — and only active immunity produces memory cells.
Immunity classifies on two independent axes: active vs passive (who made the antibodies) and natural vs artificial (how exposure happened). The four combinations have distinct mechanisms and durations. Only active immunity produces memory cells; only memory cells make subsequent responses faster.
Active and passive immunity compared.
| Immunity type | Antibodies from | Memory cells | Speed and duration |
|---|---|---|---|
| Active | Individual's own immune system | Yes — long-lasting | Slow to develop; long duration |
| Passive | Another organism (placenta, breast milk, antitoxin injection) | No | Immediate; short duration |
Vaccination is
activeimmunity, not passive. The body produces its own antibodies and memory cells in response to the harmless or inactivated antigen in the vaccine.Passivefor vaccination is an explicit reject.
Vaccination is artificial active immunity: a harmless or inactivated antigen stimulates the body to produce its own antibodies and memory cells without causing disease. Herd immunity is population-level protection that arises when enough individuals are immune that pathogens cannot find susceptible hosts; the required threshold depends on infectivity. Antigenic variation (in influenza and HIV) is why some vaccines need yearly reformulation — surface antigens mutate, and memory cells from earlier strains no longer recognise the new variants.
Monoclonal antibodies are produced from hybridoma cells and used for therapy, diagnosis, and pregnancy testing.
Monoclonal antibodies are all identical, produced from a single B cell clone and therefore all specific to the same antigen at the same complementary site. The consistency of a monoclonal preparation is not achievable with the natural polyclonal response to an infection, which is what makes them useful for medical and diagnostic work.
Immunise an animal with the target antigen so its B cells produce the desired antibody. Fuse the B cells with myeloma (cancer) cells to create hybridoma cells: the hybridoma combines the B cell's specific antibody production with the cancer cell's capacity for indefinite division. Test the hybridoma clones and culture the one producing the required antibody for large-scale production. Both parental properties must be named for full credit.
Direct therapy — antibodies bind antigens on cancer cells, marking them for immune destruction (e.g. trastuzumab for HER2-positive breast cancer). Indirect therapy (magic bullet) — a cytotoxic drug or radioactive label is attached to the antibody, which delivers it directly to cells carrying the target antigen. Diagnosis — antibodies detect specific antigens in patient samples, such as tumour markers in blood or viral antigens in tissue. Pregnancy testing — antibodies to hCG detect the hormone in urine.
HIV is an RNA retrovirus that destroys T helper cells and causes AIDS; ELISA detects HIV antigens.
HIV is an RNA retrovirus. Inside the host cell, the enzyme reverse transcriptase converts the viral RNA into DNA, which is integrated into the host chromosome. HIV infects cells carrying the CD4 protein on their surface — principally T helper cells. Progressive destruction of T helper cells causes AIDS: the immune system loses its capacity to coordinate responses, and opportunistic infections take hold.
RNA genome (two copies). DNA is an explicit reject. Reverse transcriptase and integrase, the viral enzymes carried inside the capsid. Capsid — the protein coat enclosing the genome. Capsule is rejected; capsule is a bacterial structure. Phospholipid envelope — the outer lipid bilayer derived from the host cell membrane. Cell wall and cell membrane are rejected; HIV is a virus, not a cell. Attachment proteins (gp120, gp41) embedded in the envelope. Antibodies is rejected here; antibodies are made by the host, not displayed on HIV.
HIV's genome is
RNA, notDNA. WritingDNAvoids the RNA mark even when RNA is also stated elsewhere in the answer. The retrovirus name signals which way round it goes.
Antigenic variability — HIV mutates frequently in the genes encoding surface attachment proteins, so memory cells from earlier strains do not recognise new variants. Integration into the host chromosome — once integrated as a provirus, viral DNA cannot be removed without damaging host DNA, and latent integrated virus is invisible to the immune system. Destruction of T helper cells — HIV targets the coordinators of the response needed to mount the response against HIV.
Pitfall — No HIV vaccine: antigenic variability, not antibiotic reasoning
For
why is there no HIV vaccine, lead with antigenic variability, integration into the host chromosome, and destruction of T helper cells.
HIV has no cell wallorHIV has no metabolismis the answer towhy are antibiotics ineffective against viruses— a different question testing a different mechanism. Applying the antibiotic argument to the vaccine question is an explicit reject. Both questions feature HIV; only one is answered by what HIV lacks.2024 P3 Q01.2 saw a 25% full-mark rate; most lost marks came from importing antibiotic reasoning into the vaccine answer.
- The target antigen (or antibody, depending on which is being detected) is immobilised on the test surface.
- A primary antibody binds specifically to the target via its complementary antigen-binding site; unbound antibodies are washed away.
- A secondary antibody, conjugated to an enzyme, binds to the primary antibody; unbound secondary antibodies are washed away.
- A substrate is added. The bound enzyme catalyses a reaction that produces a colour change, and the intensity of the colour is proportional to the amount of target present.
Key terms
- antigen
- antibody
- B cell
- plasma cell
- phagocytosis
- phagosome
- lysosome
- agglutination
- hybridoma
- antigen-antibody complex
- reverse transcriptase
- complementary