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Expressed on all T cells and is needed as a signal transducer for the T cell receptor

antigen processing

peptides are generated from larger polypeptides

CD8 T Cell

directly attack virus infected cell
recognize MHC I

CD4 T Cells

helper T cells
T helper 1 - recognize macrophages
T helper 2 - recognize B cells
recognize MHC II


control T cell mediated immune responses
determine the fate of transplanted tissues
HLA molecules encoded in MHC

MHC I cells

almost all nucleated cells
excluding brain and erythrocytes
process antigens from INSIDE the cell

MHC II cells

antigen presenting cells
B cells
Dendritic cells
take up antigens from OUTSIDE cell

How are MHC genes inherited?

as Haplotypes
genes are co-dominant

MHC restriction

a given T cell receptor will recognize its peptide antigen only when the peptide is bound to a particular form of MHC molecule.

T cell maturation

Start with double negative progenitor cell (lacks CD4 and CD8)
End with double positive CD3+

Positive selection

Weak recognition of MHC I/II + peptide by immature T cell
determines expression of either the CD4 or CD8 molecule

Death by neglect

Immature T-cell does not recognize MHC + peptide
undergoes apoptosis

Negative selection

Immature T cell binds too strongly to MHC + peptide
undergoes apoptosis


Adhesion molecules
allows lymphocytes to home to lymph nodes or to sites on infection


Allows T cells to "talk" to APCs

T cell activation and signal transduction

PKC activates transcription factor NKfB
Calcineurin activates transcription factor NFAT
NKfB & NFAT start production of cytokines

CD80 and CD86

molecules on APC that binds CD28 on naive t cells and deliver necessary co-stimulatory signal to t cell


delivers required second signal for T cell activation
1) Binding of TcR and CD4 to MHC class II delivers signal to T cell.
2) Clonal expansion of T cell can occur only after second signal is delivered by CD80/86:CD28 interaction.
down regulated by CTLA4


Source: APCs
Effect: inflammation


Source: Th1 cells
Effect: T cell growth

IL4 and IL5

Source: Th2 cells
Effects: antibody production, class switching


Source: Th2 (Treg)
Effects: inhibits Th1

IFN gamma

Source: Th1
EffectL inhibits Th2


Source: Th1
Effect: inflammation

CD8 T cell effector molecules

Perforin, granzymes, fas
Response: cytotoxic

CD4 T cell effector molecules

Response: down regulates Th1, inflammatory, humoral

T-helper 1 cells

Driven by IL12 and IFN-gamma
Produce IFN-gamma
Host defense against intracellular microbes
Cellular immunity (CD8 T cells, macrophages)

T-helper 2 cells

Driven by IL4
Produce IL4, 5, 10, 13 and TGF-ß
host defense against helminths
allergic reactions
Humoral Immunity (B cells)

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