Immunity to Parasites

Overview – Immunity to Parasites

Immunity to parasites involves a combination of innate and adaptive immune responses targeted at protozoa, helminths, and arthropods that exploit the host for survival. These infections are typically chronic and require a distinct immunological approach, with eosinophils, IgE, and Th2-dominant responses playing key roles in resistance and containment.

Definition

• Parasites: Organisms that live at the host’s expense, gaining nutrients without offering reciprocal benefit
• Successful parasites: Cause minimal disturbance and avoid immune recognition
• Parasite types:
  • Protozoa (unicellular)
  • Metazoa (helminths/worms)
  • Arthropods (ectoparasites)
• Tend to cause long-term infections, unlike viruses and bacteria
• Often utilise host growth factors to promote survival and reproduction

Innate Immunity

Lysozyme

• Found in tears, saliva, mucus, neutrophils
• Some parasites are susceptible to enzymatic lysis

Eosinophils

• Specialised granulocytes targeting multicellular parasites (e.g. helminths)
• Activated by:
  • T-helper cells (via IL-5)
  • Macrophages (via TNF-α, IFN-β, IL-3)

• Mechanism:
  • Fc receptors bind to parasite-bound IgE → Degranulation
  • Granule contents include:
    • Major basic protein – damages helminth cuticle
    • Eosinophil-cationic protein – toxic RNase
    • Phospholipase D, lysophospholipase
  • Respiratory burst: Generates superoxide, H₂O₂, chloride ions

Complement Activation

• Alternative & MBL (mannose-binding lectin) pathways
• Initiated by direct recognition of parasite surface molecules

Splenic Phagocytes

• Infected red blood cells displaying parasite antigens → Opsonised by antibody/complement
• Cleared by macrophages in the spleen

Adaptive Immunity

Antibodies (B-Cell Mediated)

• Most effective for extracellular stages
• IgE is the key isotype (important in helminth clearance)
• Functions include:
  • Neutralising parasite proteases
  • Blocking parasite entry routes (e.g. anal pores)
  • Inhibiting reproductive enzymes
  • Mediating Type I hypersensitivity (e.g. urticaria, asthma)

Complement Activation (Classical Pathway)

• Triggered by antigen-bound antibodies
• Can kill tachyzoites and other circulating forms

Cell-Mediated Immunity

Th1-Cell Dominant

• Targets intracellular parasites (mostly protozoa)
• Activates macrophages → Enhanced phagocytosis and intracellular killing

Th2-Cell Dominant

• Drives B-cell activation and IgE production
• The predominant response in helminth infection

Cytotoxic CD8 T-Cells

• Destroy host cells infected with intracellular parasites
• May also directly lyse larvae

Summary – Immunity to Parasites

Immunity to parasites relies on a mix of innate effectors like eosinophils and lysozymes, and adaptive mechanisms dominated by IgE and Th2 cells. Because parasites often persist chronically and employ immune-evasive strategies, the immune system must respond in both tissue-specific and systemic ways. For a broader context, see our Immune & Rheumatology Overview page.