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A parasite is an organism which lives on or in another organism called the host. The parasite needs the host to live, but the host gains no benefit from having the parasite. The 3 main classes of parasite are protozoa (unicellular organisms), worms, and arthropods (insects and arachnids).
In comparison to acute bacterial or viral infections, a parasitic infection lasts much longer due to their well-evolved and effective methods of avoiding the immune system, a parasite is successful if it can successfully avoid or subvert immune responses directed against it. Many parasites are able to survive years in a host causing little or minimal harm, however for some parasites it is beneficial to cause disease in the host.
Parasites can cause harm to the host by:
- Competing for nutrients in the host
- Disrupting host tissue
- Destroying host cells
- Mechanical blockage
Endoparasties – Live inside the host
Ectoparasites – Live outside of the host
Protozoan Parasite Immunity
Protozoa are defined as single celled, eukaryotic microorganisms that lack cell walls. Not all protozoa are parasitic however.
Innate Immunity against Protozoa
Similar mechanisms to the removal of bacteria and viruses are in place to remove protozoa threats; this includes the complement system, NK cells and phagocytosis. However, many protozoa are breed and species specific, different species can be more susceptible to different pathogens.
Acquired Immunity against Protozoa
The acquired immune response to protozoa includes both humoral (antibodies, Helper T-cells and B cells) and cell mediated responses (Cytotoxic T-cells, macrophages, NK cells and cytokines).
Antibodies specific to protozoa surface antigens are released to control parasite numbers in the blood and tissues; this is aided by TH2 (T-helper 2 cells).
The cellular mediated immunity targets the intracellular infections and is mainly TH1 (T-helper 1 cell) driven. For the effective removal of most protozoa, the combination of TH1 and TH2 aided responses are required to target the protozoa at different stages of their life cycle.
Protozoa Evasion of Immunity
The mechanisms that protozoa have evolved to avoid the immune system are:
- The avoidance of attachment and phagocytosis
- Immunosuppression of the host immune system (e.g. destruction of T cells)
- The blockage of antigen presentation (Expression in association with MHC Class II)
- Alter surface antigens (Antigenic variation)
- Block surface antigen expression to avoid detection (Parasite coats itself with host proteins)
Vaccinations to prevent possible protozoa infections have provided limited success, frequent boosters are needed and the vaccine must contain a mix of species and strains of protozoa to maximise its success.