The way in which a cell responds to contact with other cells depends on the function and activity of both cells. Most normal cells cease division if they come into contact with other cells; however, cells of the immune system interact directly with pathogens or foreign organisms to destroy them.
Under normal circumstances, most cells replicate themselves by dividing into two cells, consuming more space. When normal cells come into contact with other cells, the division process stops. This is called "cellular inhibition." This function is lost in cancerous cells, which proliferate at a dangerous rate.
Lymphatic cells and other cells involved in the immune system can be activated by contact with foreign organisms such as viruses. They attack the pathogens through a series of reactions at the cellular level.