PARASITIC

• Parasites


  Microorganisms that survive by living off of other organisms, referred to as hosts
• Types of organisms that may cause parasitic infections

  • Protozoa
  • Helminths
  • Ectoparasites
• Immune response to parasitic infections

  • Differs from that associated with bacterial infections, mostly because of the multicellular nature of parasites
• General concepts to consider in relation to host immune responses to parasites

  • Heterogeneity with respect to life cycles and antigenic expression is a key feature of parasitic agents
  • Many parasitic infections are chronic in nature
  • The mechanisms of immune evasion are significantly different from those of bacterial infections
  • Many parasites develop significant genetic and antigenic variation in a relatively short period
  • The innate immunity in the natural hosts may be genetically determined
  • Humans, as well as animals, differ widely in their ability to handle the complex antigens found in parasites
• Defenses to parasitic infection

  • Involve both innate and acquired (adaptive) immune mechanisms
• Innate or nonspecific immune response


  • May result in the destruction and removal of the parasite, thus preventing establishment of an infection
• Nonspecific immune defenses


  • Activation of cells that may destroy the parasite by phagocytosis
  • Release of cytokines such as: TNF-α, IL-1, IL-10, IL-12, type I interferons, and chemokines that enhance the immune response
  • Activation of the complement system, resulting in enhanced recognition by the immune system
• Adaptive immune responses


  • If the innate immunity is unsuccessful in eliminating the parasite, the parasite may be eliminated through activation of the adaptive immune responses, resulting in either a humoral or a cell-mediated response to the parasite
• IgE antibodies


  Play an important role in the defense against parasites such as helminths, which are too large to be phagocytized
• Killing of the parasites by ADCC (Antibody-dependent cellular cytotoxicity)

  The Fc portions of the parasite-specific IgE antibodies bind to specific receptors on the surface of eosinophils, which are then stimulated to release enzymes from their granules that destroy the parasite
• The concentration of IgE and the number of eosinophils in the peripheral blood are increased, indicating their importance in defense against parasitic infections
• Fungi


  Represent a large heterogeneous group of eukaryotic organisms that are ubiquitous in the environment, can either be considered as parasites, deriving their nutrition from living matter, or more commonly as saprophytes, living off of dead and decaying matter
• Immune defenses to fungal agents

  • Range from protective mechanisms that include innate immunity present early in the infection to specific adaptive mechanisms that are induced later
• First line of innate defense

  • Includes skin and the mucous membranes of the respiratory, gastrointestinal, and genitourinary tracts that provide physical barriers that separate the host from the environment
• Fungi possess very few factors that allow them to overcome those physical barriers; because of the nutrients and environmental conditions needed for many fungi, those mechanisms have not evolved
• Innate immune cells


  • Express various pattern-recognition receptors (PRRs) that recognize specific structures and molecules present on bacteria and fungi
• Pathogen-associated molecular patterns (PAMPs)

  Structures and molecules of the organism that are conserved among microbial species
• PRRs on the innate immune cells

  • Initiate the immune response by sensing and recognizing the presence of PAMPs present on the bacteria, fungi, or viruses