Immune response to protozoan diseases
The immune response to protozoan diseases is a complex interaction between the host's immune system and various single-celled parasitic organisms, which can lead to chronic infections. Protozoa, including species like *Entamoeba histolytica* and *Leishmania*, are known to evade the immune response through sophisticated mechanisms, complicating treatment and management. The immune system's ability to combat these infections often relies on specific immune pathways, such as the CD40-CD40L interaction, which plays a crucial role in promoting a T helper 1 (TH1) immune response.
However, protozoa can adapt to their host environments, undermining the immune system's efforts through various evasion strategies, including inhibiting complement proteins that help mediate inflammation and immune responses. This dynamic can lead to tissue damage and chronic disease, as the immune system may inadvertently cause harm while trying to eliminate the parasite. One of the most significant protozoan diseases is leishmaniasis, which poses a severe global health threat, with millions affected and a high mortality rate. Current challenges include the lack of effective treatments or vaccines, highlighting the need for ongoing research and awareness of these infections. Understanding the immune response to protozoan diseases is essential in addressing their impact on global health.
Immune response to protozoan diseases
Definition
Protozoa are simple, single-celled organisms that are part of the animal kingdom. A number of protozoan species infect humans by inhabiting the body as parasites. The parasitic protozoans of importance are species of amebas, flagellates, and sporozoan. The most recognized ameba is Entamoeba histolytica.
Immune Response
The host’s immune system interacts with the infectious organism in a vigorous way, as the host aims to eliminate the organism. Protozoa cause persistent and chronic infections, mainly because the host’s natural immunity cannot fight all protozoa. Many types of protozoa have evolved to evade specific types of immunity. Indeed, protozoa can activate dissimilar, specific immune responses, different from those of bacteria, viruses, and fungi.
Trypanosoma cruzi is an obligate intracellular parasite that invades different types of cells in vertebrate hosts, and Toxoplasma gondii can disseminate in the host because it can change and infect many nucleated cells in its path. Research indicates that cell-mediated immunity against protozoan infection is promoted by CD40-CD40L interaction. CD40-CD40L interaction is critical to the outcome of infection in a number of parasite models.
Some vital information on the paradigms of the TH (helper T cell, or T lymphocyte) immune response came from a study on immunity to Leishmania infection. Host resistance to Leishmania leads to the protozoan’s multiplication as an amastigote. This multiplication occurs within macrophages that are contingent on the polarization of the immune response to TH1. The CD40-CD40L interaction is important for promoting TH1 immune response. Numerous studies have focused on the impact of these types of interactions on the outcome of infection.
The studies on Leishmania point to a major role of CD40-CD40L interaction in distorting the immune response of TH1 for the antiparasite to defend. This interaction is crucial for a positive outcome of infection in a number of parasite models. The stimulation of CD40 proved to be useful in expanding the TH1-type response.
Protozoan parasites possess a strong ability to adapt to different host environments. They have developed many ways to corrupt the immune system’s response.
Parasite Evasion
Parasites have a variety of mechanisms to break through the immune system’s defenses. For example, complement proteins are activated when an infection is recognized. Complements are specialized proteins in the blood that act in sequence to mediate inflammation and the immune response; complements, which include cell membrane receptors and serum proteins, work to destroy infection. In leishmaniasis, the parasite would inhibit the serum proteins and cell membrane receptors by degrading the host’s vital proteins. The parasite would continue to interfere with the complement by attacking its nature to bind with the parasite membrane.
Studies have shown that immune evasion does not always lead to an increase in pathogenicity. For example, a decline of the inflammatory response, and a decrease in its severity, can occur when helminths (worms) evade the immune system.
Immunopathology
The body reacts strongly to parasites, but parasites have mechanisms of escape; sometimes they can injure the organs of the body and the immune system. These protozoan infections lead to tissue damage and disease. Chronic infection is often caused by the immune system itself, as it responds to the parasite, to antigens, and to changes to the cytokines. Some parasites can acquire a coating of what are called antigen-antibody complexes, or noncytotoxic antibodies that block the binding of certain antibodies to the parasite surface. Also, the parasitic protozoan infection can produce host immunosuppression, which will limit the host’s ability to kill the parasite. Over time, these parasites can cause lesions and organ damage. Lesions can appear on the mouth, nose, and other areas of the body. In other forms, protozoa can travel in the bloodstream and enlarge the liver, spleen, lymph nodes, and bone marrow.
Impact
Leishmaniasis is the most significant protozoan disease worldwide. Infection with the parasitic species, of which there are several found worldwide, can, in its severe form, disfigure a person and can cause death. In 2023, the World Health Organization (WHO) estimated that 700,000 to 1,000,000 new cases of leishmaniasis occur annually across the world. However, most people who are infected by parasites that cause leishmaniasis do not develop the disease. No effective medicine or vaccine exists for leishmaniasis.
Bibliography
Cohen, Jonathan, William G. Powderly, and Steven E. Opal. Infectious Diseases. 3d ed. St. Louis, Mo.: Mosby/Elsevier, 2010.
Katsambas, Andreas, and Clio Dessinioti. “Diseases Caused by Protozoa.” In Conn’s Current Therapy 2011, edited by Robert E. Rakel, Edward T. Bope, and Rick D. Kellerman. Philadelphia: Saunders/Elsevier, 2010.
"Leishmaniasis." Centers for Disease Control, 12 Jan. 2023, www.who.int/news-room/fact-sheets/detail/leishmaniasis. Accessed 3 Feb. 2025.
Lupi, O., et al. “Tropical Dermatology: Tropical Diseases Caused by Protozoa.” Journal of the American Academy of Dermatology 60, no. 6 (2009): 897-925.
Maizels, R. M. “Exploring the Immunology of Parasitism: From Surface Antigens to the Hygiene Hypothesis.” Parasitology 136 (2009): 1549-1564.