Immune Effectors in
Macrophages are one of
the main phagocytic cells capable of destroying
parasites. Once the parasite is ingested by
these cells, they begin to produce nitric oxide
(NO), a potent chemical that is toxic to these
pathogens. In fact, NO related products are
toxic to malaria, leishmania, trypanosomes,
interferon-gamma (IFN-γ) activates the macrophage
that produces the NO (Figure 3).
Direct killing by cytokine-activated
(T Helper 1) cells generates a signal (IFN-γ).
IFN-γ activates the macrophage. The macrophage
can either secrete NO to destroy the pathogen or
release the NO within its cytoplasm to directly
destroy ingested pathogens (not shown). The
arrow pointing from the parasite to the TH1 cell
indicates that it is the presence of the
parasite within the host that activates the TH1
cell. Click to enlarrge.
Antigens of many intracellular parasites are
processed and presented via the endogenous antigen-presenting
pathway. This involves ingesting the parasite,
digestion within the phagocyte's cytoplasm, and
presenting using MHC class I molecules. Antigens
presented by MHC class I molecules become
targets for cytotoxic T lymphocytes (CTL). CTL
like their targets by inserting perforin protein
into the cell membrane of the infected cell
presenting the parasitic antigen, essentially
creating a pore-like hole in the membrane. The
CTL then releasing granzymes (specialized
digestive enzymes) into the infected cell, which
induces apoptosis to occur shortly after. The
CTL detaches from its target and goes to look
for another infected cell (Figure 4).
Stages of cytotoxic T lymphocyte cytotoxicity.
Important for some protozoa's and helminths that
infect the gut (Figure 5).
Antibodies produced by the humoral part of the
immune system (B lymphocytes) bind to parasites
found in the gut. The Fc portion of the antibody
then bind to FcR (Fc receptor) on macrophages
and eosinophils which activate them to produce
and release toxic chemicals that kill the
parasite. This is an example of
antibody-dependent cellular cytotoxicity (ADCC).
Click to enlarge.
Mast cells and goblet cells
for intestinal parasites.
Mast cells release toxic mediators, while mucin secretions by goblet cells prevents
nutrient uptake by the parasite and may aid in
flushing out the parasite.
IL-4 may also induce gut motility.
important for destroying larval stages of
The following is a list of
parasites that have been profiled: