Edexcel GCSE Biology (1BI0) — Health, Disease and Medicines
A simple plastic stick can tell someone they are pregnant within minutes, but the science happening inside is surprisingly complex. Pregnancy tests rely on antibodies to detect tiny chemical changes in the body with extreme precision.
Pregnancy tests are designed to detect hCG (human chorionic gonadotropin), a hormone produced by the placenta during pregnancy. This hormone is excreted in the urine of pregnant women and acts as the specific antigen that the test is looking for.
When urine is applied to the test, it enters the reaction zone. This area contains mobile monoclonal antibodies that are highly specific to hCG.
The urine acts as a solvent and carries the mobile antibodies (along with any attached hCG complexes) up the porous nitrocellulose test strip. This movement happens against gravity through a physical process called capillary action.
Further up the strip is the test zone, which contains fixed monoclonal antibodies (also known as immobilized monoclonal antibodies) permanently attached to the test strip.
Because monoclonal antibodies are specific to a single antigen, they do not bind to other hormones in the urine. This specificity prevents false positive results.
A valid pregnancy test must always show a line in the control zone, located slightly above the test zone. This window contains different fixed antibodies that are specific to the mobile monoclonal antibodies themselves, rather than the hCG.
To create the monoclonal antibodies used in these tests, scientists rely on cell fusion. An antibody-producing B-lymphocyte is fused with a rapidly dividing tumour cell. This creates a hybridoma cell, which can both produce the specific antibody and divide indefinitely to mass-produce identical copies.
Students often say the fixed antibodies bind to the blue beads; actually, the fixed antibodies bind to the hCG hormone, which acts as a bridge to hold the bead in place.
If an exam question asks why the control line is necessary, you must explicitly state that it proves the test is 'valid' or 'functional' because it confirms the urine travelled the entire length of the strip.
When answering 6-mark mechanism questions, explicitly mention 'capillary action' as the method by which the urine moves up the strip.
Always use the precise Edexcel terminology: write 'mobile' or 'free' instead of 'moving', and 'fixed' or 'immobilized' instead of 'stuck'.
hCG (Human chorionic gonadotropin)
A hormone produced by the placenta during pregnancy and found in urine; it acts as the target antigen in pregnancy tests.
Mobile monoclonal antibodies
Antibodies attached to coloured beads that are free to move along a test strip when dissolved in a liquid like urine.
Fixed monoclonal antibodies
Also called immobilized antibodies, these are chemically glued to a specific zone on a test strip so they cannot move, acting as a trap for antigens.
Capillary action
The physical process by which a liquid moves through a porous material (like a test strip) against gravity.
Antigen-antibody complex
The structure formed when an antigen (like hCG) binds specifically to the matching binding site on an antibody.
Monoclonal antibodies
Identical copies of one type of antibody, produced from a single clone of hybridoma cells, designed to bind to one specific antigen.
B-lymphocyte
A type of white blood cell that naturally produces antibodies.
Hybridoma
A cell formed by the fusion of a B-lymphocyte and a tumour cell, used for mass-producing monoclonal antibodies.
Put your knowledge into practice — try past paper questions for Biology
hCG (Human chorionic gonadotropin)
A hormone produced by the placenta during pregnancy and found in urine; it acts as the target antigen in pregnancy tests.
Mobile monoclonal antibodies
Antibodies attached to coloured beads that are free to move along a test strip when dissolved in a liquid like urine.
Fixed monoclonal antibodies
Also called immobilized antibodies, these are chemically glued to a specific zone on a test strip so they cannot move, acting as a trap for antigens.
Capillary action
The physical process by which a liquid moves through a porous material (like a test strip) against gravity.
Antigen-antibody complex
The structure formed when an antigen (like hCG) binds specifically to the matching binding site on an antibody.
Monoclonal antibodies
Identical copies of one type of antibody, produced from a single clone of hybridoma cells, designed to bind to one specific antigen.
B-lymphocyte
A type of white blood cell that naturally produces antibodies.
Hybridoma
A cell formed by the fusion of a B-lymphocyte and a tumour cell, used for mass-producing monoclonal antibodies.
