Recognition of antigen antigen receptors

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Recognition of antigen - antigen receptors

• Dr Kathy Triantafilou
• University of Sussex
• School of Life Sciences

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What is an antigen?
Any substance capable of producing a specific
immune response
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Recognition of antigen by B and T-cells

• B-cells can recognise an epitope alone
• T-cells can recognise antigen only when it is
  associated with an MHC molecule
• There are four cell membrane molecules that are
  involved in recognition
• membrane bound antibody (B-cells)
• T-cell receptor or TCR (T-cells)
• MHC class I
• MHC class II

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What is an antibody?

• Antigen-specific products of B-cells
• Present on the B-cell surface
• Secreted by plasma cells
• Effectors of the humoral immune response,
  searching and neutralising/eliminate antigens
• Two functions
• to bind specifically to molecules from the
  pathogen
• to recruit other cells and molecules to destroy
  the pathogen once the antibody is bound to it

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Structure of the antibody molecule

• The antigen-binding region of the antibody
  molecule is called the variable region or V
  region
• The region of the antibody molecule that engages
  the effector functions of the immune system is
  known as the constant region or C region.
• They are joined by a polypeptide chain that is
  known as the hinge region

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Structure of the antibody molecule

• X-ray crystallography has revealed that the
  overall shape is roughly that of a Y
• Each arm of the Y is formed by the association of
  a light chain with a heavy chain
• The leg of the Y is formed by the pairing of the
  carboxyl-terminal halves of two heavy chains

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Light Chain

• There are two types of light chain
• kappa (k) chains
• lambda (l) chains
• No functional difference has been found between
  antibodies having l or k light chains
• In humans 60 of the light chains are k, and 40
  are l

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Heavy chain

• There are five heavy chain classes or isotypes
• IgM (m chain)
• IgD (d chain)
• IgG (g chain)
• IgA (a chain)
• IgE (e chain)
• These determine the functional activity of an
  antibody molecule

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IgG

• IgG
• most abundant immunoglobulin in the blood
• provides the bulk of immunity to most blood-borne
  infections

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IgD

• IgD
• present in low quantities in circulation
• primary function is that of antigen receptor on
  B-cells

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IgE

• IgE
• present in the serum at very low levels
• plays a role in acute inflammation and infection
  by parasites

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IgA

• IgA
• present in external secretions, such as tears,
  milk, saliva
• first line of defense against microbial invaders
  at mucosal surfaces

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IgM

• IgM
• first antibody produced and expressed on the
  surface of B-cells, also secreted
• 10 combining binding sites per molecule make it
  very effective in removal of microbes

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Recognition of antigen by T-cells

• T-cells can recognise antigen only when it is
  associated with an MHC molecule
• There are four cell membrane molecules that are
  involved in recognition
• membrane bound antibody (B-cells)
• T-cell receptor or TCR (T-cells)
• MHC class I
• MHC class II

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Role of MHC

• The Major histocompatibility complex (MHC) is a
  large cluster of genes that encodes two major
  membrane bound glycoproteins
• MHC class I
• MHC class II
• MHC molecules function as antigen presenting
  structures for T-cells

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MHC molecules

• MHC class I
• expressed on all nucleated cells
• processes endogenous antigen that is derived from
  within the cell (i.e viruses)

• MHC class II
• expressed by antigen presenting cells
• processes exogenous antigen that has been
  endocytosed or phagocytosed (from outside the
  cell)

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