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6.2.4Unit 6: Practical Skills in Biology II

Specific Immune Responses — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 10 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 5Antigens and Recognition

6.2.4 Specific Immune Responses
Self antigens
  • Antigens made by an organism’s own cells do not trigger an immune response.
Non-self antigens
  • Antigens on bacteria, viruses or transplanted organs are recognised as foreign and do trigger a response.
Presentation
  • After phagocytosis, antigen-presenting cells such as macrophages show antigens on their surface.
Binding
  • Lymphocytes with complementary receptors bind to the presented antigens and the specific response begins.
Transplants
  • Cells from a transplanted organ carry non-self antigens, which is why transplants can be rejected without treatment.

Part 2 of 5Antibody Structure

Learn the structure by linking each region of the molecule to its function.

Antibody Structure
Shape
  • An antibody is a Y-shaped protein, also called an immunoglobulin, made of four polypeptide chains.
Chains
  • Two long heavy chains are joined by disulfide bonds to two shorter light chains.
Variable region
  • The amino acid sequence differs between antibodies, forming an antigen-binding site complementary to one antigen.
Constant region and hinge
  • Constant regions are the same within a class, and the hinge gives flexibility to bind at different angles.

Part 3 of 5How Antibodies Act

1
Antibodies are secreted by plasma cells and bind to a specific antigen to form an antigen-antibody complex.
2
By binding to surface receptors, they block pathogens from attaching to and entering host cells.
3
Antibodies bind to toxins and neutralise them, in which case they act as antitoxins.
4
They cause pathogens to clump together, which is agglutination, and this limits their spread.
5
Clumped pathogens are easier for phagocytes to engulf several at a time.
6
The resulting antigen-antibody complex marks the pathogen so that phagocytes can destroy it.

Part 4 of 5Specificity

Complementary shape
  • The variable region fits only the epitope of the antigen it is complementary to, in the same way as enzyme and substrate.
Many antibodies
  • Each antibody type recognises a different antigen, so the body must produce the right type for each pathogen.
Membrane-bound
  • Some antibodies are attached to the surface of lymphocytes and act as receptors.
Secreted
  • Others are released into the blood and tissue fluid by plasma cells.

Part 5 of 5Exam Checklist & Tips

6.2.4 – Key Points for the Exam

  • Antibody A Y-shaped protein made by plasma cells that binds to a specific antigen.
  • Epitope The part of an antigen that an antibody recognises and binds to.
  • Agglutination Clumping of pathogens by antibodies.
  • Antigen-presenting cell A cell that displays pathogen antigens on its surface to activate lymphocytes.
  • Plasma cell A differentiated B cell that secretes large amounts of one type of antibody.

Common Mistakes

  • Saying that antibodies engulf pathogens; they bind, and phagocytes do the engulfing.
  • Writing that all antibodies are identical; the variable regions differ.
  • Confusing the antibody with the antigen it binds.
  • Forgetting that the antigen-binding site is complementary in shape to one antigen only.

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