Once an organism has penetrated the animal’s physical barriers to infection, a coordinated immune response is triggered. The location of pathogens can vary, with many temporarily or permanently residing outside host cells, in tissue or on epithelial surfaces. When outside cells, pathogens are susceptible to destruction by phagocytosis. When inside host cells, pathogens may be targeted by natural killer cells, which can recognize infected cells and induce apoptosis to limit the spread of infection.
To counter these responses, pathogens have evolved a range of mechanisms for evading the immune system. Ultimately, adaptive immunity provides a more targeted and robust response to pathogens that are able to evade aspects of the innate immune system.