AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Cell recognition and the immune system: Definition of antibody.
An antibody is a protein, an immunoglobulin with a quaternary structure, secreted by plasma cells derived from B lymphocytes in response to a specific antigen.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 7 of 18
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An antibody is a protein, an immunoglobulin with a quaternary structure, secreted by plasma cells derived from B lymphocytes in response to a specific antigen. Its binding sites have a tertiary structure complementary to that one antigen, so binding is highly specific and forms an antigen-antibody complex. For example, a monoclonal antibody designed to locate a bacterial membrane protein will only bind to that specific protein because its tertiary structure is complementary to it. It is crucial to remember that an antibody is not an enzyme and catalyses nothing; therefore, it has a binding site, not an active site.
What earns the marks
- An antibody is a protein or immunoglobulin with a quaternary structure.
- The binding site has a tertiary structure complementary to that antigen, forming an antigen-antibody complex.
- Distinguish membrane-bound antibody on a B cell from secreted antibody released by a plasma cell.
Common mistakes and how to put them right
Calling the antibody's binding site an active site.
Put it right: Use 'binding site' and avoid enzyme vocabulary.
Saying antibodies kill or destroy pathogens directly.
Put it right: Binding leads to agglutination, neutralisation or enhanced phagocytosis.
Swapping antigen and antibody within one sentence, which reverses the biological meaning.
Put it right: The antigen is the foreign molecule; the antibody is the protein that binds it.
Attributing antibody production to T cells or to phagocytes.
Put it right: Plasma cells derived from B lymphocytes secrete antibodies.
Stating that B lymphocytes secrete antibodies.
Put it right: B cells carry membrane-bound antibody; plasma cells secrete antibody.
Exam tips
- Write the definition as four distinct clauses (protein, plasma cell origin, specific antigen trigger, complementary binding site) to ensure completeness.
- Never import enzyme vocabulary; antibodies have binding sites, not active sites, and they do not show induced fit.
Try the lesson
The start of the Cell recognition and the immune system: Definition of antibody. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement defines an antibody?
Listen
An antibody is a protein, an immunoglobulin with a quaternary structure, secreted by plasma cells derived from B lymphocytes in response to a specific antigen. Its binding sites have a tertiary structure complementary to that one antigen, so binding is highly specific and forms an antigen-antibody complex. For example, a monoclonal antibody designed to locate a bacterial membrane protein will only bind to that specific protein because its tertiary structure is complementary to it. It is crucial to remember that an antibody is not an enzyme and catalyses nothing; therefore, it has a binding site, not an active site. A strong answer earns marks for these points. An antibody is a protein or immunoglobulin with a quaternary structure. The binding site has a tertiary structure complementary to that antigen, forming an antigen-antibody complex. Distinguish membrane-bound antibody on a B cell from secreted antibody released by a plasma cell. A common slip: Calling the antibody's binding site an active site; antibodies are not enzymes and have binding sites. Instead, use 'binding site' and avoid enzyme vocabulary. Exam tip: Write the definition as four distinct clauses (protein, plasma cell origin, specific antigen trigger, complementary binding site) to ensure completeness.
See it
Key words
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antibody
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plasma cells
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complementary binding site
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antigen-antibody complex
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See every definition
- antibody
- A protein or immunoglobulin with a quaternary structure, secreted by plasma cells derived from B lymphocytes in response to a specific antigen.
- plasma cells
- Cells derived from B lymphocytes that secrete antibody.
- complementary binding site
- A binding site with a tertiary structure complementary to one antigen, so binding is highly specific and forms an antigen-antibody complex.
- antigen-antibody complex
- The complex formed when the binding site, complementary to that antigen, binds it.
Label the diagram
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1Box 1
2Box 2
3Box 3
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