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The adherence in vitro of leucocytes to the surface of various stages in the life cycle of T. spiralis and N. brasiliensis in the presence of serum was examined. Considerable differences were observed in the behaviour of mast cells, eosinophils, neutrophils and macrophages in this interaction. Mast cells adhered for a short time, did not flatten onto the surface and did not degranulate. Adherence ceased after 4-6 hr. Eosinophils adhered within minutes to the surface or worms, flattened and degranulated; only their cytoplasmic remnants could be seen on the worms' surface after 24 hr in culture. In contrast, only a small area of the cytoplasmic membrane of neutrophils flattened on the surface of the worms and adherence ceased after 2-24 hr. The NBT conversion reaction showed a positive deposit at the interface between neutrophils and parasites during neutrophil adherence. This deposit remained as "foot prints" on the surface of the nematodes following neutrophil detachment. This positive NBT reaction occurred only with neutrophils and not with eosinophils, mast cells or macrophages. Macrophages adhered permanently to the surface of these worms, they did not flatten and retained their integrity. Under the light microscope the cytoplasmic inclusions appeared to decrease in size during culture. Electron microscopy revealed the presence of fewer granules and an increased number of vacuoles in later cultures of macrophages. These findings are discussed in relationship to the immunopathology of nematode infection in vivo.

The major red cell sialoglycoproteins, the glycophorins, play a central role in the invasion of human red cells by Plasmodium falciparum. En(a-) cells deficient in glycophorin A (alpha) and S-s-U- cells deficient in glycophorin B (delta) are relatively resistant to invasion, while trypsin treatment of S-s-U- cells, which removes most of the remaining sialoglycoprotein, renders these cells almost totally resistant to invasion. Parasites inside these glycophorin-deficient cells develop normally. Invasion of erythroid precursors in vitro by merozoites of P. falciparum parallels the appearance of glycophorins on the surface of these nucleated cells, even though parasites fail to develop inside them. However, another type of cell from an erythroleukaemic line (K562) which expresses glycophorins on its surface is resistant to invasion. Furthermore, the observed increased invasion of young cells as opposed to an older cell population is not related quantitatively to the presence of glycophorins on the cell surface. Thus, although the role of glycophorins is both specific and important in the invasion of cells by P. falciparum, it is clearly only part of a complex process.

The nature of complement binding to the surface to infective larvae of Trichinella spiralis and Nippostrongylus brasiliensis differs. When worms were incubated in serum from uninfected hosts, washed and incubated in fluorescent reagent the whole surface of T. spiralis fluoresced but in N. brasiliensis fluorescence was confined to the anterior end and some internal organs. The outer structure of the cuticle of the T. spiralis larvae was shown not to contain ATP-ase, thus differing from many cell membranes.

Several observations suggest that iron is essential for the development of malaria parasites but there is evidence that the parasites in erythrocytes do not obtain iron from haemoglobin. The total haemin level in parasitized erythrocytes does not vary during parasite development, indicating that the iron-containing moiety of haemoglobin is not detectably metabolized. Although parasite proteases can degrade the protein part of haemoglobin in red cells, no parasite enzymes that degrade haemin have been identified. In mammalian cells, haemin is degraded to carbon monoxide and bilirubin by the enzyme haeme oxygenase. This enzyme has not been found in malaria parasites. In fact haemin has been found to be toxic to parasite carbohydrate metabolism. Thus, iron apparently cannot be liberated from haemin and instead is sequestered in infected red cells as haemozoin, the characteristic pigment associated with malarial infection. If iron bound to transferrin is the source of ferric ions for malaria parasites within mature erythrocytes, then the parasite must synthesize its own transferrin receptor and localize it on the surface of the infected cell, because the receptors for transferrin are lost during erythrocyte maturation. Our results here suggest that Plasmodium falciparum synthesizes its own transferrin receptors enabling it to take up iron from transferrin by receptor-mediated endocytosis.

The red cell membrane appears to possess receptors for malarial parasites which are species specific. Plasmodium falciparum invades red cells that have the surface sialoglycoproteins, glycophorins A, B and C. Several regions of these molecules are critical to parasite binding. Invasion of red cells by merozoites can be blocked by both antibodies directed to specific sites on glycophorin and tryptic fragments of these molecules. The parasites appear to bind to the red cells in a lectin-like fashion, since three monosaccharides, namely N-acetyl-glucosamine (Glu NAc), N-acetyl-galactosamine (Gal NAc) and N-acetyl-neuraminic acid (Neu NAc), can specifically block parasite invasion in vitro. Neoglycoproteins made by coupling these sugars to BSA are particularly effective. Possible mechanisms of parasite attachment to and invasion of red cells are discussed.

Newborn larvae of Trichinella spiralis were collected for 30 min from female worms in culture, incubated in vitro for various times up to 18 h, and surface-labelled with iodine. The detergent-solubilised products were examined by SDS-polyacrylamide gel electrophoresis. At time periods up to 6 h these larvae expressed only one Mr 64 000 iodine-labelled surface protein. Some time between 6 h and 18 h a further three components (apparent Mr 58 000, 34 000 and 32 000) became accessible to surface labelling. All four of these components are antigenic in that they can be immunoprecipitated with T. spiralis immune sera. Tryptic peptide analysis revealed that the 32 and 34 kDa antigens were structurally very similar, but the 58 and 64 kDa proteins differed from each other and the 32-34 kDa pair. Thus T. spiralis not only undergoes a total change in surface antigens between moults, but also major changes in surface antigen expression within one stage.