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Most of the organisms of the phylum Nematoda are free living, but some are animal or plant parasites of major importance to man. During their life cycle all nematodes undergo a series of moults in which they shed an external cuticle, consisting of an outermost membrane-like layer of unknown composition and a series of fibrillar layers similar to collagens. Because of this structure, the cuticle has been viewed as an acellular exoskeleton with rather inert molecular components. However, observations have shown that it contains enzymes and sometimes haemoglobin, and that nutrients are absorbed through it in the infective larvae and adut stages of Brugia pahangi. It is bound by complement and antibody, resulting in the adherence of leukocytes, and antibody-dependent cell-mediated reactions damage the cuticle of newborn larvae of Trichinella spiralis and the microfilariae of Dipetalonema viteae and Litomosoides carinii. We report here that the surface of the cuticle of the parasitic nematode Trichinella spiralis expresses protein molecules which change qualitatively following the moulting process, and quantitatively during growth of the worms within one stage. Also, surface proteins are released in vitro at a rate which depends on the conditions of culture of the worms.

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The surface antigens of Toxocara canis infective larvae have been identified by radio-iodination and compared with the excretory-secretory (ES) products released by the larvae in vitro. Common antigens, of molecular weight 32 000 and 120 000 are found on the larval surface, in the ES material and in culture supernatant following surface iodination of living T. canis larvae. The 120 000 antigens consist of three closely migrating bands in each of these preparations. However, one prominent ES component, of molecular weight 400 000, is not found on the larval surface. Additional molecules of 55 000 and 70 000 are present in the ES material, but while these may be discerned in surface preparations there appears to be more heterogeneity of surface molecules in this size range. Both sets of molecules are antigens to infected patients and experimental animals. A comparison of characterized human sera show that a radio-immunoprecipitation assay correlates with the established ELISA test (r = 0.89), and that all labelled molecules are antigenic to the infected host.

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Antibody responses were measured in a volunteer infected four times with Necator americanus over a 27-month period. The main source of antigen was culture fluid in which living adult N. americanus had been maintained for several days. Antibodies to worm acetylcholinesterase and IgE antibodies were detected only with this material, but antibodies were identified by the enzyme-linked immunosorbent (ELISA) assay, with either adult worm secretions or extracts of third-stage infective larvae. The total serum IgE level fell after the first infection, but although it then increased during subsequent infections, it never rose above 600 U per ml. None of the antibody responses suppressed the rat of worm development to maturity, or reduced the fecundity of the parasites. However, it is suggested that the development of the immune response may be associated with the waning of the severe gastro-intestinal symptoms which were experienced in this infection, and which are frequently characteristic of hookworm infections.

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