6.8 In carbo communication 173 BOX 6.3 DEATH RECEPTORS (DEATH SENSORS) Perhaps the most intriguing types of cell sensors are death receptors, which participate in the cell's suicide mechanisms in multicellular organisms of animals, including humans. The cell suicide mechanism, called apoptosis, allows control of the cell number in tissues and elimination of those individual cells, which may threaten the survival of the living organism. Failure of a cell to commit suicide may result in cancer, while early suicide of the cells contributes to severe disorders such as Parkinson's and Alzheimer's diseases [28]. Certain cells have special sensors, which detect the presence of extracellular death signals and, in response, switch on the cell's internal suicide process [28]. 6.8 IN CARBO COMMUNICATION Single-cell organisms can send external signals to communicate with one another and even with higher organisms by several mechanisms, such as: (i) chemical communication via signal molecules; (ii) optical communication via bioluminescence; and (iii) `tactile' communication via direct contact. 6.8.1 Chemical cell-to-cell communication