Resistance to the protozoan parasites involves nonspecific factors as well as specific factors such as humoral and cellular mechanisms. Cellular immunity is believed to be the most important defense mechanism in diseases such as leishmaniasis and toxoplasmosis.
Macrophages play an important role in animals infected with Toxoplasma. Cytokines are involved in the control of both the immune response and pathology.
The cytokines produced by T cells and other immune cell types do not act directly on the parasites but influence other cell types of host, to control both immune response and pathology.
The response of immune cells to cytokines includes a variety of physiological changes, such as changes in glucose, fatty acid and protein metabolism that usually influence all phases of immune response.
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Unlike most viral and bacterial infections, protozoan diseases are often chronic, lasting months or years. Protozoan disease when associated with a strong host immune response, the chronic infection is apt to result in a high incidence of immunopathology.
The extra cellular stay of protozoans can elicit humoral responses. Antigen-antibody complexes in the region of antibody excess activate hageman blood coagulation factor (Factor XII), which in turn activates the coagulation, fibrinolytic, kinin and complement systems.
This type of immediate hypersensitivity is responsible for various clinical syndromes including hyperviscosity of blood, edema, and hypotension etc. in African trypanosomiasis.
Similar disease mechanisms involving a strong humoral immune response would be expected in other infections by protozoans.
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In general antibodies serve to control the level of parasites existing in the blood stream and tissue fluids, whereas cell mediated immune responses are directed largely against intracellular parasites.
Serum antibodies directed against protozoan surface antigens may opsonize, agglutinate or immobilize them to kill with the help of complement and cytotoxic cells. The immune complexes circulating in serum get deposited in the kidneys and other tissues of humans and animals infected with protozoans. If inflammatory cell infiltrates accompanied these deposits, it causes glomerulonephritis.
Kinin System
The plasma kinin system is an enzymatic system that is triggered by activated factor XII.
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The activation of prekallikrein into kallikrein by the activated factor XII occurs on the surface of biomaterials and in the fluid phase (Figure 4).
Fibrinolytic mechanism:
It is an ordered sequence of interactions between an enzyme and a cofactor on a specific substrate; the components of the process include plasminogen, tissue plasminogen activator, urokinase and kallikrein.
Some of the antibodies may act to inhibit protozoan enzymes in such a away that the pathogens replication is prevented.
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For example T. brucei gambiense lives within the bloodstream (bloodstream form) to reenter its vector through blood. Its presence in the blood induces humoral immune response and the protozoan’s may be phagocytyzed by macrophages. But many protozoans are resistant to phagocytic process and may even replicate within macrophages.
In infections by Leishmania species, cellular defense mechanisms depends upon CD4 + T-lymphocytes and activated macrophages as effector cells that are regulated by cytokines of Th1 subset. But many protozoans are resistant to humoral and cell mediated immune responses due to the defense mechanisms adopted by them.