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438 ParT ThrEE Host Defenses to Infectious Agents

ability to “autoinfect,” can maintain its lifecycle for decades. the infection, helminths appear to reflect a harmonious host–
Chronic infections certainly reflect an adaptation that leads to parasite interface such that relatively asymptomatic carriers are
“parasitism,” as mortality induced in the host would prevent available as reservoirs for ongoing transmission. Of course, failure
parasite transmission if the host died before larval release or egg to establish this harmonious coexistence does occur, leading to
production could occur. In addition to the long-lived nature of pathological conditions exemplified by cirrhosis and portal
hypertension in schistosomiasis and elephantiasis associated with
lymphatic filariasis.
Schistosoma mansoni
> 200 million infected
Echinococcus, Taenia spp PROTOTYPICAL HOST RESPONSES TO HELMINTHS
>100 million infected
The canonical host immune response to all helminths is of the
T-helper 2 (Th2) type and involves the production of cytokines
interleukin (IL)-4, IL-5, IL-9, IL-10, and IL-13; the antibody
Brugia malayi Trematodes Cestodes isotypes immunoglobulin G1 (IgG1), IgG4, and IgE; and expanded
populations of eosinophils, basophils, mast cells, type 2 innate
Onchocerca volvulus lymphoid cells, and alternatively activated macrophages (Chapter
2
Wuchereria bancrofti Ancylostoma duodenale
157 million infected Nematodes Nector americanus 16). However, it is also being increasingly recognized that while
576 million infected the predominant response is Th2 in nature, a large regulatory
component involving both regulatory cytokines and cells are
3
also part of this repertoire. The Th2 response induced by
helminth parasites is quite stereotypical, but its initiation, progres-
sion, and culmination of this response requires interaction with
many different cell types, most notably (i) epithelial/stromal
cells, (ii) innate lymphoid cells (ILCs), (iii) dendritic cells (DCs)
and macrophages; (iv) T cells; (v) B cells; (vi) eosinophils; (vii)
Ascaris lumbricoides
>1 billion infected Trichinella spiralis mast cells/basophils; and (viii) neutrophils (Chapter 2). In addi-
Trichuris trichiura tion, the host–helminth interactions can lead to a variety of
>600 million infected modulated immune responses that are mediated largely by the
FIG 31.1 Common and medically relevant helminth infections induction of regulatory T cells (Tregs) and alternatively activated
and their global prevalence. macrophages (AAMs) (Fig. 31.3).

A B

C D
FIG 31.2 The clinical manifestations of lymphatic filariasis, including (A) mild lymphedema,
(B) severe lymphedema, (C) elephantiasis, and (D) hydrocele.