Figure 1.

A schematic representation of the senses that are hardwired in the brain. The senses that protect the individual from external and internal perturbations through a contact delivery of information to the brain include the five senses, the proprioception, and the seventh sense—immune input. The peripheral immune cells detect microorganisms and deliver the information to the brain. Although neurons are the primary targets, when an excess of immune information is delivered (in pathology), microglia respond either as bystanders or as active players. (Note that arrows schematically indicate inputs of senses into the brain circuits but not the precise position of where each sense is being projected.)