T-celactivering en klonale selectie

Naive T cells express membrane proteins called the cluster of differentiation, or CD markers with the T cell receptor or TCR.

There are two major T cell subpopulations expressing either CD4 or CD8 proteins.

When interacting with an antigen, the CD4 T cells recognize MHC II-antigen complexes, and CD8 T cells bind antigens loaded onto MHC I molecules.

In either case, the T cells receive a secondary signal from one of the costimulatory molecules.

For example, in CD4 T cells, after the TCR binds an MHC II-antigen complex, the CD86 molecules on dendritic cells additionally bind the CD28 on T cells, enabling the two cells to adhere longer.

Such co-stimulation confirms the initial activation signal, preventing T cells from mistakenly attacking normal tissues.

The activated T cells from both sub-populations undergo clonal selection, dividing to form a pool of clones that can recognize the specific stimulating antigen.

Further, these clones give rise to two subtypes of highly specialized cells called effector and memory cells.