Monoclonal Antibodies

Recall from 4.3.1.3 that an antibody is a Y-shaped protein made by lymphocytes that binds to one specific antigen. In the body, dozens of slightly different antibodies are made against any one pathogen because lots of different lymphocytes recognise it. That's good for fighting infection, but it's no good if you want a clean, reproducible diagnostic test or therapeutic.

A monoclonal antibody ("mono" = one, "clone" = identical copies) is a population of antibodies that are ALL identical because they all came from the same clone of cells. Every molecule binds to the same binding site on the same antigen — like a million identical keys all fitting one type of lock.

That specificity is what makes mAbs so useful. A blood test that uses a monoclonal antibody will only react with the molecule the antibody was designed to recognise — pregnancy hormone, prostate-cancer marker, a particular virus. No false positives from related molecules.

Why we need a special production method. A normal lymphocyte makes the right antibody but, like all body cells, can only divide a limited number of times. It can't be kept in culture indefinitely. Tumour cells, on the other hand, divide forever (they don't stop) but they don't make the antibody you want. Scientists solved this by fusing the two — combining the antibody-making power of the lymphocyte with the immortal-division power of the tumour cell.

This combined cell — a hybridoma — was first made by César Milstein and Georges Köhler in Cambridge in 1975 (Nobel Prize 1984). UK biotech grew out of this discovery, and monoclonal antibodies are now a multi-billion-pound industry that produces NHS diagnostics and major cancer drugs.

AQA mark schemes for this topic are very prescriptive — they want a clear, step-by-step description. Memorise these stages in order:

Step 1 — Choose the antigen. Decide what you want the antibody to bind (e.g. the pregnancy hormone hCG, or a protein on the surface of cancer cells).

Step 2 — Inject a mouse with the antigen. The mouse's immune system responds: B-lymphocytes that recognise the antigen multiply and make antibody.

Step 3 — Remove lymphocytes from the mouse's spleen. The spleen is rich in active B-lymphocytes. These cells make the right antibody but cannot divide indefinitely in culture.

Step 4 — Fuse lymphocytes with tumour (myeloma) cells. Tumour cells divide rapidly and forever. By fusing the two cell types (using a chemical or electric pulse that opens the cell membranes briefly), each fused cell — a hybridoma — has:

• the antibody-making genes of the lymphocyte;
• the indefinite-division ability of the tumour cell.

Step 5 — Select and grow the hybridomas. Only successfully fused cells are kept. Each hybridoma is allowed to divide many times in culture to form a clone — every cell in the clone is genetically identical and makes the same antibody.

Step 6 — Collect and purify the antibody. The clone secretes its antibody into the surrounding culture medium. The antibody is filtered out, concentrated and purified.

Result: large quantities of identical antibody, all binding to the same antigen at the same binding site. These can then be used in diagnostic tests, cancer treatments, research and more (4.3.2.2).

Why the fusion is needed.

Cell type	Makes antibody?	Divides indefinitely?
Normal B-lymphocyte	✅ specific antibody	❌ limited divisions
Tumour cell (myeloma)	❌ no specific antibody	✅ divides forever
Hybridoma (fused)	✅ specific antibody	✅ divides forever

The hybridoma is the best of both — it can be cultured in industrial bioreactors, producing milligrams to kilograms of identical antibody.

Ethical considerations. AQA expects you to be aware of some downsides:

• Animal use. The starting lymphocytes come from a mouse, which must be killed to remove its spleen. The UK regulates this under the Animals (Scientific Procedures) Act 1986; researchers need a Home Office licence and must show the work cannot be done without animals.
• Side effects in humans. Early monoclonal antibodies were made entirely from mouse proteins. The human immune system recognised them as foreign and produced antibodies against the antibody — causing side effects and reducing effectiveness. Modern engineering produces humanised or fully human monoclonal antibodies that are tolerated better, but unexpected reactions can still occur.
• High cost. Manufacture is complex, so monoclonal antibody drugs (e.g. trastuzumab/Herceptin for breast cancer) are expensive. NHS commissioning bodies (NICE in England) decide whether the benefits justify the cost.

These ethical and practical points are popular for 4-mark and 6-mark exam answers — be ready to write about both the science AND the issues.

Home pregnancy tests sold by UK pharmacies (Boots, Superdrug) and the NHS all work the same way. They use monoclonal antibodies specific to hCG (human chorionic gonadotrophin), a hormone the developing placenta releases into a pregnant woman's urine and blood from about a week after conception.

How a typical stick test works:

1. The user wees on the absorbent end of the stick.
2. Urine soaks up into a reaction zone containing mobile monoclonal antibodies attached to coloured beads (often blue or pink). These antibodies are specific to hCG.
3. If hCG is present, it binds to the mobile coloured antibodies and the whole complex moves along the strip.
4. Further along the strip is a test line containing a second set of monoclonal antibodies, fixed in place, also specific to hCG (but binding a different site on the hormone).
5. The hCG–coloured-antibody complex sticks to this fixed line, building up enough coloured beads to make a visible coloured line — the positive result.
6. A control line further along contains antibodies that bind ANY of the mobile antibodies, regardless of whether they carry hCG. It always appears and shows the test has worked.

If there is no hCG in the urine, the coloured antibodies flow past the test line without sticking — no test line appears, only the control line (negative result).

This is why pregnancy tests are reliable, fast and cheap: the antibodies are monoclonal, so they bind ONLY hCG and not any other urine molecule — no false positives from related hormones.

Use 2 — Diagnosis in laboratories. Many NHS blood tests work like the pregnancy test in principle: a monoclonal antibody binds the target molecule and a signal (colour, fluorescence) is produced. Examples:

• Cancer markers — e.g. PSA (prostate-specific antigen) test for prostate cancer screening.
• Infection diagnosis — e.g. antibody tests for HIV, hepatitis, COVID-19 lateral-flow tests.
• Hormone levels — e.g. thyroid hormone, fertility hormones.

Because the antibody is monoclonal, it binds only the target molecule — the test is highly specific. Multiple antibodies can be combined to detect several diseases from one blood sample.

Use 3 — Research and locating specific molecules. Scientists tag monoclonal antibodies with a fluorescent dye. When the labelled antibody is added to a cell or tissue sample, it binds only the target molecule. Under a fluorescence microscope, the dye lights up — showing exactly where the molecule is.

UK research uses include:

• Mapping which proteins are expressed in which cells of the brain (Cambridge, MRC LMB).
• Showing how viruses enter cells (Oxford virology labs).
• Identifying cancer cell types from a biopsy.

This is also how forensic labs identify specific substances in tiny samples — fingertip drug residues, for example.

Use 4 — Treating diseases. This is where mAbs have transformed UK NHS oncology over the last 20 years. Three approaches are common:

(a) Carrying a drug or radioactive substance to cancer cells.

A monoclonal antibody is selected that binds only to a molecule on the surface of cancer cells (e.g. HER2 protein on some breast cancers). A drug, radioactive isotope or toxin is chemically attached to the antibody. When the antibody is injected into the patient, it travels through the bloodstream and binds only to cancer cells — delivering its payload directly to the tumour while leaving healthy cells alone.

(b) Blocking growth signals.

Some cancer cells grow because a receptor on their surface receives constant 'grow' signals from the body. A monoclonal antibody designed to fit that receptor blocks the signal — the cancer cell stops dividing. Herceptin (trastuzumab) is the famous UK example: it binds HER2 and is approved by NICE for some breast cancers.

(c) Triggering the immune system to destroy the cancer.

Some mAbs mark the cancer cell so the patient's own immune system attacks it. Newer 'checkpoint inhibitor' mAbs (e.g. pembrolizumab — Keytruda) take the brakes off T-cells so they can destroy tumours.

Other treatment uses outside cancer:

• Infliximab for inflammatory bowel disease (UK NHS).
• Adalimumab for rheumatoid arthritis.
• Omalizumab for severe asthma.

Evaluation — why scientists were over-optimistic at first.

When mAbs were first developed, hopes were that they would be a 'magic bullet' — targeted with no side effects. In practice:

• Pro: much more specific than traditional chemotherapy, so often gentler.
• Con: still many side effects (flu-like symptoms, low blood pressure, allergic reactions).
• Con: early antibodies were mouse-derived and the patient's immune system attacked them. Modern engineering (humanised mAbs) reduces this but does not eliminate it.
• Con: very expensive to manufacture — single courses can cost the NHS tens of thousands of pounds per patient. NICE decides whether a treatment is cost-effective enough to be funded.
• Con: they don't work for every patient — only those whose cancer expresses the target molecule (e.g. only HER2-positive breast cancers respond to Herceptin).

AQA examiner reports stress that 'monoclonal antibodies have not been used as widely as scientists hoped' — be ready to explain WHY (mostly the side-effect and cost issues above).