Antigen-presenting cells are cells that express surface MHC (Major histocompatibility complex) class II molecules. MHC class II molecules preferentially bind to T cell receptors associated with the CD4 surface molecule. Thus, APCs present antigen to the CD4+ T cells, the helper/inducer subset.
Class I MHC molecules preferentially bind to T cell receptors associated with the CD8 surface molecule. Class I MHC molecules are present on the surface of all nucleated cells, thus allowing cells to present their internal antigens to cytotoxic T cells.
This mechanism is critically important in host defense against intracellular pathogens such as viruses.
T-cell activation requires two signals:
- Engagement of the T-cell receptor by the antigen/MHC complex
- A second signals, usually transduced by direct cognate interaction (“touching”) between constimulatory cell surface molecules on the APC and T cell.
T-cell activation resulting from the two signals leads to production of cytoplasmic and nuclear factors, resulting in gene activation and new DNA synthesis.
