CD40 is a costimulatory protein of the TNF-receptor superfamily that is expressed on the membrane of antigen presenting cells and is involved in their activation. It is a vital regulator of numerous inflammatory and immune responses and functions in germinal center formation and memory B cell development. It is also required for amyloid-beta-induced microglial activation and is thought to be involved in the pathogenesis of Alzheimer’s disease. Absence of CD40 leads to Hyper-IgM syndrome type 3. In immunohistochemistry of normal tissue, CD40 has membranous and extracellular positivity in a wide range of cell types, including: B cells, dendritic cells, mast cells, monocytes, platelets, macrophages, endothelial cells, epithelial cells, fibroblasts, keratinocytes, Langerhans cells, nevus cells, pancreatic islet cells, prostatic acini, astrocytes and microglial cells.
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