Adaptive Immunity to Parasites

Overview

The role of the adaptive immune system in parasitic infections is mainly to orchestrate the innate immune reactions to the parasites by producing a specific immune response. This could be done either through the cell-mediated immune response or through the humoral antibody response. An important note is that the information given here is generalised as the response to each individual parasite is different as parasites themselves range from parasitc worms to malaria (for more information see the Parasitology section of WikiVet).

Cell-Mediated

Although the innate immune system provides an effective first line of defence, T cells are fundamental in the development of immunity, demonstrated using T-cell deprived mice that fail to resolve otherwise non-lethal infections of, for example, T. cruzi.

• Both CD4⁺ and CD8⁺ cells are required for protection, e.g CD4⁺ cells protect against the blood stage of a Plasmodium infection (erythrocytes do not express MHC class I), while CD8⁺ cells are required to mediate immunity against the liver stage (hepatocytes do not express MHC class II).
• T_H1 cells are required to fight intracellular protozoa - the release of Interferon-γ (IFNγ) activates macrophages to kill the protozoa residing within them
• Helminth infections require both T_H1 and T_H2 responses, e.g. during S. mansoni the secretion of IFNγ by T_H1 cells activates mechanisms that destroy larvae in the lungs, although the T_H2 subset, secreting Interleuken-5 (IL-5), predominate. IL-5 is the cytokine responsible for the eosinophilia associated with parasite infections.
• T_H2 cells are required for the destruction of intestinal worms, where they induce mucosal mast cells and interact with eosinophils

Humoral

While cell-mediated immunity is important in tissue infections, such as Leishmania, specific antibodies are important in controlling parasites that live in the bloodstream, e.g. malaria. Mechanisms of antibody-mediated immunity include:

• Directly damaging protozoa
• Activating complement, and the subsequent Membrane Attack Complex
• Blocking attachment to host cells
• Enhancing macrophage phagocytosis
• Involvement in antibody-dependent cell-mediated cytotoxicity

The humoral response to parasitic worms involves the production of IgE. It is stimulated by mast cells producing IL-4 (innate response). It also includes isotype switching of B cells to produce IgE.

Also see Immunity to Parasites

Originally funded by the RVC Jim Bee Award 2007