A Framework for Human Host Immune Responses to Four Types of Parasitic Infections

Human host immune responses to parasitic infections are complex. They can be categorized into four immunological pathways against four types of parasitic infections. For intracellular protozoa, the eradicable host immunological pathway is TH1 immunity involving macrophages, interferon gamma (IFNg) CD4 T cells, innate lymphoid cells 1 (ILC1), CD8 T cells, invariant natural killer T cells 1 (iNKT1) cells, and immunoglobulin G3 (IgG3) B cells. For free-living extracellular protozoa, the eradicable host immunological pathway is TH22 immunity involving neutrophils, interleukin (IL)-22/IL-17 CD4 T cells, innate lymphoid cells 3 (ILC3), iNKT17 cells, and IgG2 B cells. For endoparasites (helminths), the eradicable host immunological pathway is TH2a immunity with inflammatory eosinophils (iEOS), IL-5/IL-4 CD4 T cells, IL-25 inducing inflammatory innate lymphoid cells 2 (iILC2), mast cells-tryptase (MCt), iNKT2 cells, and IgG4 B cells. For ectoparasites (parasitic insects and arachnids), the eradicable host immunological pathway is TH2b immunity with inflammatory basophils, mast cells-tryptase/chymase (MCtc), IL-3/IL-4 CD4 T cells, IL-33 inducing nature innate lymphoid cells 2 (nILC2), iNKT2 cells, and immunoglobulin E (IgE) B cells. The tolerable host immunity against ectoparasites and endoparasites is TH9 immunity with regulatory eosinophils, regulatory basophils, IL-9 mast cells (MMC9), thymic stromal lymphopoietin inducing innate lymphoid cells 2, IL-9 CD4 T cells, iNKT2 cells, and IgA2 B cells. This categorization provides a complete framework of immunological pathways against four types of parasitic infections.