Unlike most other types of infection, protozoan diseases are often chronic, lasting for m onths to years. When associated with a strong host immune response, this type of long term infection is apt to result in a high incidence of immunopathology.
IMMUNE RESPONSE
Unlike most other types of infection, protozoan diseases are often chronic, lasting for m
onths to years.
When associated with a strong host immune response, this type of long term
infection is apt to result in a high incidence of immunopathology. Until
recently the importance of host immune response in controlling many parasite
infections was not fully appreciated, but the impact of HIV infection on many
parasitic diseases has highlighted this relationship.
Different parasites
elicit different humoral and/or cellular immune responses. In malaria and
trypanosome infections, antibody appears to play a major role in immunity,
although it would seem that for many organisms both humoral and cellular
immunity are required for killing of parasites. Cellular immunity is believed
to be the most important mechanism in the killing of Leishmania and Toxoplasma.
Cytokines are involved in the control of both the immune response and also the
pathology of many parasitic diseases. Helper (h) and cytotoxic (c) T cells play
major roles in the induction/control of the response. The various subsets of
these produce different profiles of cytokines. For
example, the Th1 subset
produces gamma interferon (IFNγ)
and interleukin-2 (IL-2) and is involved in cell-mediated immunity. In
contrast, the Th2 subset produces IL-4 and IL-6, and is responsible for
antibody-mediated
immunity. The induction
of the correct T-cell response is key to recovery. The Th1 subset and increased
IFN-γ are important for the control of Leishmania, T. cruzi and Toxoplasma infections,
whereas the Th2 response is more important
in parasitic infections in which antibody is a major factor. It is important to
recognize that the cytokines produced by one T-cell subset can up or down regulate
the response of other T-cell subsets; IL-4 will down regulate Th1 cells for example. The cytokines
produced by T and other cell types do not act directly on the parasites but
induce changes in the metabolism of glucose, fatty acid and protein in other
host cells. Cytokines can also stimulate cell division and, therefore, clonal
expansion of T and B-cell subsets. This can lead to increased antibody
production and/or cytotoxic T-cell numbers. The list of cytokines and their
functions is growing rapidly, and it would appear that these chemical messages
influence all phases of the immune response. They are also clearly involved in
the multitude of physiological responses (fever, decreased food intake, etc.) observed
in an animal’s response to a pathogen, and in the pathology that results.
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