A patient receives radioactive monoclonal antibodies that bind specifically to antigen T on cancer cells. A scanner shows a hotspot only when enough radioactivity is concentrated in one region. Small groups of cancer cells are spread across an organ. The antibodies bind to these cells, but the scan shows no hotspot.
Which explanation is correct?
Each small group of cancer cells holds too little bound radioactivity for the scanner to detect.
The scanner detects only radioactive antibodies that are still free in the blood, not bound ones.
The radioactive label separates from each antibody once the antibody has bound to antigen T.
The antibodies bind just as much to healthy cells, so the cancer cells are not more radioactive.
87 exam-style questions on Edexcel GCSE Biology 5.3 Immune system and medicines, covering 5.3.1 The specific immune system, 5.3.2 Response to immunisation, 5.3.3 Advantages and disadvantages of immunisation, 5.3.4 Antibiotics and bacterial infections, 5.3.5 Aseptic techniques in culturing, 5.3.6 Cross-sectional areas of cultures, 5.3.7 Stages of developing new medicines, and 5.3.8 Production of monoclonal antibodies. Each one has a worked solution and a mark scheme showing where the marks go.