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. References: Immunology. 16: 111–35, PMID: 9597126; CNS Neurol Disord Drug Targets. 2010 Apr;9(2):149-55, PMID: 20205645; J Neuroinflammation. 2006 Feb 24;3:3, PMID: 16504112; Diabetologia. 2005 Feb;48(2):268-76, PMID: 15690148