| Themes > Science > Life Sciences > General Biology > Immunology > Immunity to Infection > Different immune effectors protect against different pathogens |
The immune system has to cope with a spectrum of pathogens which have distinct lifestyles and obviously different arms of the immune system are needed in different situations. On top of this many pathogens have evolved specific counter measures which limit or inhibit the effectiveness of the immune response. We will consider both the type of immunity required to eliminate an first infection and the protection developed against subsequent challenge. We can divide the type of pathogen up as follows:
Extracellular organismsBacteria. These are probably the simplest type of organism to combat and in many cases the innate immune system may be able to clear an infection using complement and phagocytosis. Specific antibody is highly effective, both by directing complement lysis and inducing opsonisation and phagocytosis. Some bacteria have evolved capsules which prevent recognition by innate mechanisms and require both antibody and complement opsonisation to promote efficient clearance by phagocytes. Where toxins are produced antibody is of course vital.Protective immunity is essentially humoral, with IgA playing an important role in organisms that infect mucosal surfaces (respiratory tract, gut, genito-urinary tract). Parasites Large, multicellular parasites present a special problem to the immune system and indeed are rather poorly eliminated. The mechanisms deployed include antibody directed complement attack and ADCC, in particular by eosinophils. Innate immunity is generally ineffective. The parasites employ many evasion strategies including complement inhibitors, release of large quantities of soluble antigen (decoy) and acquisition of host proteins. Intracellular organismsBacteria and Protozoa Many bacteria have evolved resistance to the constitutive killing mechanisms used by phagocytes. These pathogens actively replicate inside bacteria, either in the phagosome or, in some cases where a specific adaptive mechanism has been acquired, in the cytoplasm. This type of bacteria cannot be eliminated by immediate immune mechanisms. Sometimes innate immunity can be effective, for example Listeria Monocytogenes can be eliminated by macrophages activated by NK cells, TNFalpha and IFNgamma are required for this (probably also IL12 but I am not aware this has been shown). More frequently T cell activation is required and a TH1 response necessary for clearance of the organism. Antibody is generally ineffective in eliminating a first infection. Cytotoxic T cells also play a role in clearing many intracellular organisms. Memory T cells are the key players in protecting against most intracellular bacteria and parasites. Antibody is occasionally though rarely protective. Viruses Viruses are a very diverse group of obligate intracellular pathogens. Almost every form of immunity comes into play against some type of virus. Enveloped viruses can be damaged by complement attack, and some directly bind C1q or homologous collectins. Phagocytes can take up and destroy antibody and complement coated viruses. However the key players in antiviral immunity are interferon, NK cells, antibody, CTL and TH1 cells taken in temporal order in a first encounter. |
| Protection against subsequent challenge varies with the behaviour of the virus but antibody is highly effective in preventing reinfection if it is of the right type and against the appropriate epitope. Antibody which prevents infection of a susceptible host cell by a virus is termed neutralising. This property requires that the antibody either prevent binding of the virus to its' receptor or blocks some reaction necessary for viral entry. Because of the slower response time, T cell memory is rarely able to prevent the establishment of a secondary infection but is of considerable importance in limiting its' spread. |
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