IBA1 is an acronyn for “ionized Calcium binding adapter molecule 1”, and the protein is also known as AIF1 for “allograft inflammatory factor 1”. AIF1 was originally identified, cloned and sequenced as a protein heavily upregulated in an animal model of graft rejection (1). The AIF1 protein was localized in macrophages and neutrophils surrounding and infiltrating the graft site. Shortly afterwords the same protein was identified a gene product which had some interesting properties, including Calcium binding and the important observation that IBA1 was only expressed in hematopoietic cells (2). IBA1 and AIF1 were subsequently found to be identical, a small globular 17kDa molecule belonging to the “EF” hand superfamily of Calcium binding proteins. Since the only hematopoietic cells and in the neuropil of the central nervous system are microglia, suitable IBA1 antibodies are widely used to identify microglial cells in sections and tissues (3). In tissue samples from which they have not been washed out by perfusion, lymphocytes within blood vessels are also IBA1 positive. Microglia are the immunocompetent cells of the CNS and are extremely important in responses to injury and disease. Microglial are small but very active cells which constantly send processes probing their neighborhood and which alter morphology and are induced to divide following a variety of CNS compromises (4). Many important and highly cited papers have made use of IBA1 antibodies as markers of microglia (e.g. 5,6). The IBA1 antibody was made against the C-terminal peptide of human IBA1 coupled to keyhole limpet hemocyanin. It works well on western blots, on cells cultures and sectioned material.