Antigen presenting cells

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MHC

  • aka Major histocompatibility complex
  • All cells in the body express Class I MHC molecules
  • Antigen presentation to T cells is done by MHC molecules
  • Two classes of MHC are required (Class I and Class II) because there are two different types of antigens; there are a total of four classes (Class I, Class II, ClassIb and Class III)
  • If one desires a specific response, this is a good way to get the differentiation
  • The MHC does not do anything, and falls off if there is no peptide attached to it.
  • Coded by MHC genes
  • MHC genes are downloaded from the parental units
  • MHC genes are used in paternity litigation

Classes

Class I

  • HLA-A, -B, and -C
  • Expressed on the surface of all nucleated cells
  • Present peptide antigens to CD8+ T cells
  • Present endogenous peptides
  • Show great polymorphism
  • Has a groove at the top to accept the molecule (located in the α/heavy chain)
  • Variability exists on the top of the groove and in the groove
  • Has one polymorphic membrane MHC chain (α) and one invariant non-transmembrane chain (&beta2-microglobulin)
  • Immunoglobulin-like domains: α1, α2

Class II

  • HLA-DR, -DP, -DQ and also DM and DO
  • Expressed on the surface of professional APCs
  • Present peptide antigens to CD4+ T cells
  • Present exogenous peptides
  • Show great polymorphism
  • Has the exact same structure as the Class I molecules
  • Only difference is that Class II molecules have two transmembrane proteins
  • Immunoglobulin-like domains: β1, β2

Class Ib

  • aka Class I-like
  • CD1
  • Show some polymorphism

Class III

  • includes some genes important for immune functions such as tumour necrosis factors α and β
  • Show little or no polymorphism
The T cell recognition of antigens is MHC restricted

MHC restriction

  • My immune system is programmed to listen to my MHC (self MHC)
  • 10% can see foreign MHC (Allo recognition)
  • This is the mechanism that drives transplant rejection
  • More stringent requirements for a peptide to fit in, since the consequences are more severe

HLA

  • aka human leukocyte antigens
  • These are the human MHC


APCs

  • Antigen Presenting Cells
  • Required for antigen processing and presentation
  • These steps are required to activate T cells
  • Characterized by the expression of MHCs
  • present in the context of either Class I or Class II MHC molecules

Types

Dendritic cells

  • Defined by their shape (they look like dendrites)
  • Present in all cells
  • Concentrated in lymph nodes, lungs, respiratory and GI tracts, and under the skin

Macrophages

  • not as good as Dendritic cells

Activated B cells

  • Is (as the name implies) an activated B cell
  • Has to be activated (cannot be inactivated)
  • Not as good as macrophages (worst of the three types)