The use of monoclonal antibodies in: targeting medication to specific cell…

The idea

Monoclonal antibodies are identical antibodies produced by a single clone of plasma cells. Each molecule has the same variable region and binds one specific antigen. In targeted medication, a cytotoxic drug is attached to an antibody whose binding site is complementary to an antigen present in greater numbers on target cells, like a cancer cell protein. The antibody carries the drug directly to these cells and binds, forming an antigen-antibody complex. The drug then kills the target cells. A smaller dose is needed and healthy cells are affected less, reducing side effects. In medical diagnosis, the antibody carries a label rather than a drug, such as an enzyme, fluorescent dye or radioactive marker, allowing the specific antigen to be located.

What earns the marks

• Monoclonal antibodies being identical, and specific to one antigen, because they come from one clone of cells.
• The binding site being complementary to an antigen found on, or in greater numbers on, the target cells.
• The drug being attached to the antibody and so delivered to those cells, with an antigen-antibody complex forming.
• The consequence, that a lower dose can be used and there are fewer side effects on healthy cells.
• The antibody carrying an enzyme, dye or radioactive label in a diagnostic test, so the antigen can be located or measured.

Common mistakes and how to put them right

• Saying the antibody kills the cancer cell, when the attached drug does so after the antibody has bound.

• Claiming the target antigen is found only on cancer cells, when it is usually present in far greater numbers there.

• Describing the binding as an active site or as an enzyme-substrate reaction.

• Leaving out why side effects are reduced, which is a key part of explaining the advantage of targeted medication.

• Using the word monoclonal without saying that all the molecules are identical.

Exam tips

• Answer in three moves: the antibody is specific to an antigen on the target cell, the drug is attached to the antibody, so only target cells receive the drug.
• In diagnosis questions name the label and say what makes it visible, colour change, fluorescence or radioactivity.
• Use data from the stem about antigen numbers rather than asserting that only diseased cells carry the antigen.