How does the immune system identify pathogens displayed by antigen-presenting cells?

The immune system identifies pathogens primarily through the recognition of specific antigens that are presented on Major Histocompatibility Complex (MHC) molecules by antigen-presenting cells (APCs). APCs, such as dendritic cells, macrophages, and B cells, play a crucial role in immune surveillance by processing and presenting peptide fragments of pathogens (antigens) on their surface bound to MHC molecules.

When T cells encounter these MHC-antigen complexes, they utilize their T cell receptors (TCR) to recognize and bind specifically to the presented antigens. This recognition is critical for the activation of T cells and subsequent immune responses, including the activation of other immune cells, such as B cells and cytotoxic T cells.

The interaction between TCRs and MHC-antigen complexes is highly specific, allowing the immune system to distinguish between self and non-self, as well as between different types of pathogens, which is essential for mounting an effective immune response. Thus, the role of MHC molecules in displaying specific antigens is fundamental to the immune system's ability to recognize and respond to infections.