Which type of proteins are expressed by Antigen Presenting Cells (APCs)?

Antigen Presenting Cells (APCs), such as dendritic cells, macrophages, and B cells, play a crucial role in the immune response by processing and presenting antigens to T cells. The proteins that are key to this function are Major Histocompatibility Complex (MHC) molecules.

MHC class I molecules are present on almost all nucleated cells and are responsible for presenting endogenous antigens (those synthesized within the cell) to CD8+ cytotoxic T cells. On the other hand, MHC class II molecules are primarily expressed by professional APCs and are designed to present exogenous antigens (those that are taken up from outside the cell) to CD4+ helper T cells. This interaction is essential for the activation and regulation of the adaptive immune response.

The presence of both MHC class I and MHC class II in APCs allows these cells to effectively communicate and engage with both CD8+ and CD4+ T cells, leading to a robust immune response. Hence, this characteristic of APCs highlights their fundamental role in linking the innate and adaptive arms of the immune system. This is why the correct answer is the expression of both MHC class I and MHC class II proteins.