How MHCII signaling promotes benign host-microbiota interactions

The major histocompatibility complex (MHC) is a hyper-polymorphic gene-dense region found on Chromosome 6 in humans (the human MHC is termed the HLA for "human leukocyte antigen"). The "hyper"-polymorphic nature of this region stems from the extreme allelic diversity found within classical Class I and class II MHC (MHCII) genes 1 (Fig MHCII genes encode cell-surface glycoproteins that bind extracellularly derived peptide antigens and present them on the surface of antigen presenting cells (APCs; conventionally, dendritic cells DCs, macrophages, and B cells). MHCII:peptide complexes engage T cell receptors (TCRs) and CD4 co-receptors which facilitates cognate interactions between CD4 + T cells and APCs.