Which component of the immune system aids in distinguishing between self and non-self antigens?

The Major Histocompatibility Complex (MHC) plays a crucial role in the immune system's ability to differentiate between self and non-self antigens. MHC molecules are found on the surface of cells and are essential for presenting antigens (protein fragments from proteins that are part of pathogens or other foreign entities) to T cells, which are pivotal for adaptive immunity.

When a pathogen infects a cell, fragments of its proteins are processed and displayed on the cell's surface in conjunction with MHC molecules. There are two main classes of MHC molecules: MHC class I is present on all nucleated cells and presents endogenous antigens (from within the cell), while MHC class II is found on professional antigen-presenting cells (like dendritic cells, macrophages, and B cells) and presents exogenous antigens (from outside the cell).

T cells, particularly cytotoxic T cells and helper T cells, interact with these MHC-antigen complexes. If a T cell recognizes a peptide presented by an MHC molecule as foreign, it activates the immune response against that particular antigen. Conversely, if the T cell encounters a self-antigen presented by the MHC, it typically undergoes negative selection to prevent an autoimmune reaction.

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