Welcome to First Signal

The obligatory first step for triggering a CD4+ or CD8+ T cell response requires a successful binding and presentation of antigenic peptides of the groves of major histocompatibility complex (MHC)class I and class II molecules. MHC class I is expressed on all nucleated cells, whereas MHC class II expression occurs on professional antigen-presenting cells (APCs) however it can be conditionally expressed in all cell types. These molecules play the pivotal role in diverse conditions. During organ transplant, MHC molecules provoke immune response leading to transplant rejection. Malignant neoplastic cells alter MHC expression to successfully evade an anti tumor immune response. Further, studies indicate the association of major diseases with polymorphic alleles of MHC genotypes.