Pathogens can be found in various compartments

Infections can be intracellular or extracellular. Intracellular infections can be within the cytoplasm or within vesicles, which are located in the cytoplasm. Examples of the former: viruses, Chlamydia, rickettsia, listeria etc. Examples of the latter: mycobacteria, salmonella, listeria, Legionella etc. How does the body control the infections within the cytoplasm? This is done mainly via CD8 T cell activation and differentiation into CTLs that directly kill the infected cell, NK cells that respond by directly killing the infected cells as well. How does the body control the infections that are within vesicles? T cells recognise these cells and then kill them, CD4+ cells get activated and produce cytokines that activate macrophages and B cells (which produce antibodies and kill infected cell). NK cells kill the infected cell but also produce a macrophage activating cytokine – IFN-γ.

Extracellular infections can occur in interstitial places, blood, and lymph. Some examples of pathogens that do this are: viruses, protozoa, fungi, Helminths, bacteria etc. The main way the immune response reacts to this sort of infection is by humoral immunity (antibody protection), phagocytosis and neutralisation. Extracellular infections can also occur within epithelial surfaces. Many examples of microbes cause this (i.e.: candida, strep, staph etc). Ig A antibodies are produced in immunity, and inflammatory cells are recruited to the site of infiltration.

Category: Pathology Notes