Immunology – effector cells against helminths (large extracellular parasites) In host defense, which leukocyte type is primarily responsible for killing large parasites such as helminths via extracellular degranulation (e.g., major basic protein, eosinophil cationic protein)?

Introduction / Context:
Helminths are multicellular parasites that are too large to be phagocytosed. The immune system therefore relies on extracellular killing mechanisms. Understanding which leukocytes specialize in anti-helminth activity is high-yield in immunology and parasitology.

Given Data / Assumptions:

• Target organisms are large extracellular worms (e.g., Ascaris, Schistosoma).
• Question asks for the cell type that kills by releasing toxic granule contents onto the parasite surface.
• Consider classical antibody-dependent mechanisms (IgE/IgG) and Th2 cytokines (IL-5).

Concept / Approach:

Eosinophils bind to IgE- or IgG-coated helminths via Fc receptors and release toxic proteins (major basic protein, eosinophil cationic protein, eosinophil peroxidase) and reactive oxygen species. IL-5 from Th2 cells drives eosinophil maturation and activation. This extracellular degranulation damages parasite integument and facilitates expulsion.

Step-by-Step Solution:

Verification / Alternative check:

Eosinophilia is a hallmark laboratory finding in tissue-invasive helminth infections; anti-IL-5 therapies reduce eosinophil counts and can diminish helminthic cytotoxic responses, supporting the mechanism.

Why Other Options Are Wrong:

• Basophils: important for immediate hypersensitivity and IgE support but not principal helminth killers.
• Monocytes: differentiate to macrophages; mainly phagocytose smaller targets.
• Neutrophils: key for bacteria/fungi; limited role against large helminths.
• NK cells: kill virus-infected or tumor cells, not specialized for helminths.

Common Pitfalls:

• Assuming “more granules” means better for all pathogens; granule content specificity matters.
• Confusing basophils with eosinophils in Type II/III vs. Type I hypersensitivity contexts.

Final Answer:

Eosinophils