LECTURE #8 – T CELL DEVELOPMENT
After studying lecture #8, you should be familiar with the following concepts.
1. The T Cell Receptor (TCR). T cells detect foreign antigen through the TCR. Each
TCR is a heterodimer composed of an α and a β chain. Each chain contains a variable
and a constant region. The variable region is responsible for antigen recognition. The
TCR is permanently anchored in the plasma membrane and has no effector functions.
Similar to the BCR, a TCR is generated through somatic recombination. V, J and C gene
segments form the α chain and separate V, D, J and C gene segments form the β chain.
Upon successful formation of an α chain at one locus recombination is shut off at the
other α locus. This is referred to as allelic exclusion and it ensures that each T cell
expresses a single α chain. Allelic exclusion also occurs at the β chain locus and during
2. TCR – MHC interactions. The TCR interacts with antigenic peptides presented by
MHC molecules (either class I or class II). The α chain and the β chain of the TCR
interact with the peptide. Additional TCR residues interact with the MHC. The variable
region of the TCR mediates TCR-MHC interactions. Stable formation of a TCR/MHC
complex requires additional interactions of CD4 or CD8 on the T cell with the presenting
MHC molecule. CD4 interacts with an invariant region of MHC II molecules and CD8
interacts with invariant regions of MHC I molecules. Thus, the class of cell presenting the
antigen (i.e. MHC I or MHC II) determines the nature of the responding T cell (CD4 or
3. CD4 and CD8 T cells. CD4 T cells are also referred to as helper T cells. They interact
with other immune cells such as B cells or macrophages and drive their activation. CD8
T cells are also known as