Antigen presenting cells

From IKE

Contents 1 MHC 1.1 Classes 1.1.1 Class I 1.1.2 Class II 1.1.3 Class Ib 1.1.4 Class III 1.2 MHC restriction 1.3 HLA 2 APCs 2.1 Types 2.1.1 Dendritic cells 2.1.2 Macrophages 2.1.3 Activated B cells

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

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

• There are some "professional" APCs, and some that are "non-professional"
• Any nucleated cell could be a non-professional APC
• Three types of professional APCs: dendritic cells, macrophages and activated B cells

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)