Every antibody has the same basic shape, but the tips of its two arms differ from one antibody to the next. That variation is what lets the body, and a hospital laboratory, target one molecule out of millions.
An antibody is a globular glycoprotein made by plasma cells. It has four polypeptide chains, two long heavy chains and two short light chains, held together by disulfide bonds. The tips form two variable regions whose antigen-binding sites are complementary to one specific antigen; the constant region is the same for a class of antibodies and binds to receptors on phagocytes; a hinge region gives flexibility. Antibodies cause agglutination, neutralise toxins and viruses and mark pathogens for phagocytosis (opsonisation). Monoclonal antibodies, identical antibodies from one clone, are made by the hybridoma method: B cells from an immunised mouse are fused with myeloma cells, and the hybridoma making the wanted antibody is selected and cloned. Because they bind one antigen, they are used to diagnose disease (pregnancy tests, locating cancers) and to treat it (targeting cancer cells).
With the structure chip on, click each part of the antibody: the heavy chains (blue), light chains (green), variable regions (orange), constant region, hinge and disulfide bonds. Then switch to agglutination, neutralisation and opsonisation.
| Part | Structure | How it relates to function |
|---|---|---|
| variable regions | tips of both heavy and light chains; amino acid sequence differs between antibodies | form two antigen-binding sites with a shape complementary to one antigen, so binding is specific; two sites let one antibody bind two antigens |
| constant region | same amino acid sequence for all antibodies of a class | binds to receptors on phagocytes (and can activate other defences) |
| hinge region | flexible part of the heavy chains | lets the arms move so the antibody can bind antigens different distances apart |
| disulfide bonds | covalent bonds between cysteine R groups | hold the four chains together in a stable quaternary structure |
The antigen-binding site is in the variable region, made by both a heavy and a light chain. Do not put the binding site on the constant region, and do not call the variable region the "active site": that term is for enzymes.
Step through the chips 1 immunise, 2 fuse, 3 screen and 4 clone. Then look at the two uses: diagnosis and treatment.
| Step | What happens | Why |
|---|---|---|
| 1 immunise | a mouse is injected with the antigen | B cells making the matching antibody multiply in its spleen |
| 2 fuse | B cells from the spleen are fused with myeloma (cancer) cells, using polyethylene glycol or an electric pulse | a B cell makes the antibody but cannot divide in culture; a myeloma cell divides indefinitely. The hybridoma does both |
| 3 screen | hybridomas are grown one per well and tested | only the cells making the wanted antibody are kept |
| 4 clone | the selected hybridoma divides by mitosis in a fermenter | every cell makes identical antibody, which is collected and purified: monoclonal antibodies |
A hybridoma is a fusion of a B-lymphocyte (or plasma cell) with a myeloma cell. Answers that fuse "a B cell with a T cell" or "two plasma cells" lose the mark.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the antibody sim, which part has a shape complementary to one specific antigen?
In the agglutination view, why do the bacteria end up clumped together?
In step 2 of the monoclonal sim, why are B-lymphocytes fused with myeloma cells?
In the diagnosis view of the monoclonal sim, what does the control line show?
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
How many polypeptide chains does an antibody molecule have?
Which feature of an antibody allows phagocytes to recognise a pathogen coated with antibodies?
During the production of monoclonal antibodies, why are the hybridoma cells screened?
Why are monoclonal antibodies useful for delivering drugs to cancer cells?
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