Defence Against Pathogens

Antigens and Antibodies

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Ben Whitten

Learning Objectives

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What are antigens?
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Antibody-mediated immunity and cell-mediated immunity are both triggered by antigens, which are any substance capable of causing a specific immune response. They are large molecules such as proteins, carbohydrates, lipids, or nucleic acids and may include things such as virus particles, whole micro-organisms (such as a bacterial cell), part of a bacterium (such as the flagella, cell wall or capsule), toxins, molecules on cells such as blood cells, pollen grains and egg whites.


Large molecules produced in a person’s own body, called self-antigens, do not cause an immune response. Foreign compounds that do trigger an immune response are non-self antigens

The immune system becomes programmed prior to birth to distinguish between self-antigens and non-self antigens, and from then on, it only attacks non-self antigens.

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What are antibodies?
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An antibody is a Y-shaped specialised protein that is produced by plasma cells in response to a non-self antigen. Antibodies belong to a group of proteins known as immunoglobulins, often represented as ‘Ig’.


There are five classes of antibodies, which vary in their structure and are designated;


  • IgA

  • IgD

  • IgE

  • IgG

  • IgM



The antibody produced in response to an antigen can combine with that antigen to form an antigen-antibody complex. Each antibody can combine with only one particular antigen.

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What are antigen-presenting cells?
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When a non-self antigen enters a body, specific cells recognise this and respond appropriately. These cells are called antigen-presenting cells, and include dendritic cells, macrophages and undifferentiated B-cells – these cells:


  • Detect the presence of a non-self-antigen

  • Engulf the pathogen

  • Digest the pathogen, producing small fragments that move to the surface of the cell

  • Present the antigen to lymphocytes

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External Defences
Protective Reflexes
Phagocytosis
Inflammatory Response
Fever
Lymphatic System
The Immune Response
Antigens and Antibodies
Antibody-Mediated Immunity
Cell-Mediated Immunity
Types of Immunity
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