Chapter 8 - Biology of the T Lymphocyte

The T-cell receptor complex (TcR complex) is shown in figure 8.4.  What is shown is the TcR and its associated protein molecules called CD3:

The actual antigen-specific receptor is called the TcR and it is composed of an a-chain and a b-chain.  These are integral membrane proteins which are associated much like an Fab of the antibody molecule Ð Each chain has a V-region and a C-region.

Associated with the TcR is a group of integral membrane proteins with transmembrane signaling domains.  This group of proteins is collectively called CD3.  This complex is responsible for transducing signals into the T-cell when antigen connects with the TcR.

Two other molecules (called co-receptors) are also associated with the TcR complex: CD4 and CD8.

Mature T-cells either one or the other, not both.  These T-cell populations are thus called CD4(+) T-cells or CD8(+) T-cells. 

These co-receptor molecules control how the T-cell interacts with the antigen-presenting cell or (APC) as shown in figure 8.3:

CD4(+) T-cells associate with class II MHC molecules.  CD4(+) T-cells are also known as helper T-cells or T_H cells.

CD8(+) T-cells associate with class I MHC molecules.  CD8(+) T-cells are also known as cytotoxic T-cells or T_C cells.

In addition to these molecules, figure 8.5 describes the array of other surface molecules and receptors that are important for T-cell function.

CD 28 binds to B7 on B-cells Ð this acts as a second signal for T-cell activation (co-stimulation)

Conversely, CTLA-4 (CD152) sends an inhibitory signal when it binds to B7.

CD40L (CD154) binds to CD40 on B-cells to help induce class (isotype) switch.