Catalysis of the phosphorylation of a histidine residue in response to detection of an extracellular signal such as a chemical ligand or change in environment, to initiate a change in cell state or activity. The two-component sensor is a histidine kinase that autophosphorylates a histidine residue in its active site. The phosphate is then transferred to an aspartate residue in a downstream response regulator, to trigger a response. [ http://www.ncbi.nlm.nih.gov/pubmed/9191038 http://www.ncbi.nlm.nih.gov/pubmed/20223701 http://www.ncbi.nlm.nih.gov/pubmed/10966457 ]