Elements consistent with a cell mediated immune response were identified immunohistochemically in amyotrophic lateral sclerosis (ALS) spinal cord and Alzheimer disease (AD) hippocampus. T helper/inducer and cytotoxic/suppressor lymphocytes were detected in affected tissues in both diseases. In addition, abundant reactive microglia were found expressing the major histocompatibility glycoproteins HLA-A,B,C and HLA-DR, as well as receptors for the Fc chain (Fcyγl), for complement 3 and 4, and for vitronectin. In AD, the complement proteins Clq, C4d, C3d and C5b-9 were found on dystrophic neurites, neuropile threads and some neurofibrillary tangles. In ALS, the only complement proteins identified were C4d and C3d. The integrin ligands vitronectin and ICAM-1 were also identified in affected tissues in both diseases.