Bacteria

• When the body detects harmful antigens, such as bacteria, it triggers an immune response and secretes killer T cells, which destroy cells through the process of lysis, to neutralize the bacteria.
  
  

Viruses

• Unlike bacterial infections, which exist in the form of an overabundance of bacterial cells, viral infections cause harm when viruses infect and inhabit the body's natural cells, injecting their DNA into the cells and reconfiguring their function to produce more of the virus. Helper T cells mark cells infected with a virus, and then the killer T cells eliminate the infected cells through the secretion of cytotoxins.
  
  

Tumor Cells

• Tumor cells--whose mutated DNA allows it to focus solely on growth and reproduction at the expense of natural functions--occur naturally and seldom cause harm, though an especially rapid rate of expansion can indicate cancer. Killer T cells lyse such cells before they reproduce at too harmful a rate.
  
  

Immune Response

• Killer T cells also limit and down-regulate immune responses. When the cells eliminate the catalyst of the immune response--such as bacterial or viral infection or tumor growth--the body must stem the spread of cells secreted by the immune system to reduce the chance that they will turn on other, non-harmful cells. Killer T cells suppress the response before it turns harmful.
  
  

Memory Cells

• Though most killer T cells die after the body has staved off infection, some remain in the bloodstream as long-lived memory cells, poised to quicken the immune response if the infection or attack recurs. This parallels how immunization works: The introduction of a small dosage of an antigen to the immune system allows the body to easily overcome it and its respective memory cells to remain in the body.