Parasitic Antigens and Immunity

Parasites evoke strong immune responses in their hosts. Understanding parasitic antigens and the types of host immunity is crucial for disease prevention, control, and vaccine development.

I. Antigens from Parasites

Parasites present a variety of antigenic components at different life stages. These antigens are targets for the host’s immune system and are also used in diagnostic tests and vaccine development.

1. Cuticle or Tegument

• Definition:
  • The outermost covering of parasites, especially nematodes (cuticle) and trematodes/cestodes (tegument).
• Importance:
  • Major source of surface antigens.
  • Often stimulates strong innate immune responses.
• Examples:
  • Cuticle proteins of Ascaris lumbricoides.
  • Tegumental antigens of Schistosoma spp.

2. Subcellular Fraction

• Definition:
  • Antigens derived from specific organelles or structures inside the parasite.
• Example:
  • Flagellar antigens of Trypanosoma spp., important for immune recognition.

3. Excretory-Secretory (E/S) Antigens

• Definition:
  • Metabolic byproducts and secretions actively released by parasites during their life in the host.
• Importance:
  • Highly immunogenic.
  • Often used in diagnostic tests because they reflect active infection.
• Examples:
  • E/S products of Toxocara canis larvae.
  • Secretory products of Fasciola hepatica.

4. Eggs

• Definition:
  • Eggs produced by parasites can themselves be highly antigenic.
• Importance:
  • Antigens from eggs play a role in chronic inflammation and tissue damage.
• Examples:
  • Eggs of Schistosoma mansoni causing granuloma formation.

5. Larva

• Definition:
  • Antigens prepared from the larval stages of parasites.
• Importance:
  • Larval stages often stimulate strong immune responses as they migrate through host tissues.
• Examples:
  • Larval antigens of Toxocara canis causing visceral larva migrans.

6. Moulting Fluid

• Definition:
  • Fluid released during the moulting (ecdysis) process of parasites (especially nematodes).
• Importance:
  • Contains unique antigenic proteins.
• Examples:
  • Moulting fluid antigens of filarial worms like Wuchereria bancrofti.

II. Types of Immunity Against Parasites

The host immune system fights parasitic infections using various types of immunity, which can be either innate (natural) or acquired (developed after exposure).

1. Innate Immunity

• Definition:
  • Non-specific, naturally present defense mechanisms against parasites.
• Key Features:
  • Immediate response.
  • Includes physical barriers (skin, mucosa), phagocytic cells (macrophages, neutrophils), and natural killer (NK) cells.

2. Acquired Immunity

• Definition:
  • Specific immunity developed after exposure to parasitic antigens.
• Subtypes:

a. Active Immunity

• Definition:
  • Immunity resulting from direct exposure to antigens via infection or vaccination.
• Examples:
  • Immunity after coccidiosis infection in poultry.

b. Passive Immunity

• Definition:
  • Immunity resulting from the transfer of antibodies from an immune host to a non-immune host.
• Examples:
  • Maternal antibodies transferred via colostrum.

c. Humoral Immunity

• Definition:
  • Antibody-mediated immunity primarily involving B lymphocytes.
• Key Features:
  • Neutralizes extracellular parasites and their toxins.
  • Antibodies like IgG, IgM, IgE play roles in defense against parasites.

d. Cell-Mediated Immunity

• Definition:
  • Immunity involving T lymphocytes, macrophages, NK cells, and other immune cells.
• Key Features:
  • Important against intracellular parasites.
  • Includes activation of cytotoxic T cells and macrophage activation.
• Examples:
  • Immunity against Leishmania spp. (intracellular protozoa).

e. Sterilizing Immunity

• Definition:
  • Immunity that persists even in the absence of the parasite.
• Key Features:
  • Provides complete protection against reinfection.
• Examples:
  • Immunity following recovery from coccidiosis.

f. Premunity

• Definition:
  • Low-level infection maintained to sustain immunity; immunity wanes if parasite is eliminated.
• Key Features:
  • Immunity persists only in presence of a few parasites.
• Examples:
  • Seen in Babesia and Theileria infections in cattle.

g. Cross Immunity

• Definition:
  • Immunity against one parasite that confers protection against another closely related species.
• Key Features:
  • Important in parasite control strategies.
• Examples:
  • Immunity against Fasciola hepatica may protect against amphistomes.

h. Concomitant Immunity

• Definition:
  • Host develops immunity against newly invading stages but cannot eliminate existing established parasites.
• Key Features:
  • Protects the host from further infection but allows existing parasites to survive.
• Examples:
  • Seen in chronic Schistosoma infections.

Summary

Component	Type/Antigen	Key Features	Examples
Parasitic Antigens	Cuticle/Tegument	Surface antigens	Schistosoma, Ascaris
	Subcellular Fraction	Specific organelles	Trypanosoma flagellar antigen
	Excretory-Secretory	Metabolic byproducts	Fasciola, Toxocara
	Eggs	Highly antigenic	Schistosoma eggs
	Larvae	Antigens from larval stages	Toxocara canis
	Moulting Fluid	Unique proteins during moulting	Filarial worms
Host Immunity	Innate	Non-specific defense	Physical barriers, NK cells
	Active	From antigen exposure	Post-infection immunity
	Passive	From antibody transfer	Maternal antibodies
	Humoral	B-cell mediated	Antibody responses
	Cell-mediated	T-cells, macrophages	Leishmania defense
	Sterilizing	Lasting after parasite clearance	Coccidiosis recovery
	Premunity	Maintained by live parasites	Babesia, Theileria
	Cross Immunity	One parasite protects against another	Fasciola and amphistomes
	Concomitant Immunity	Immunity against new but not existing parasites	Chronic Schistosoma