Parasitology and Immune Responses

Host-parasite and immune responses

A host-parasite relationship is decided by how the host's defences recognise and respond to the invader. Immunity has two broad lines. The first line of defence consists of surface barriers such as intact skin and mucous membranes, which physically block entry of pathogens. Pathogens that breach these barriers meet the second line, a nonspecific innate system made of complement proteins, interferons, and phagocytic cells. Adaptive immunity is specific and has memory. Active acquired immunity arises after the host itself meets an antigen, through infection or vaccination. Passive natural immunity is conferred when maternal antibodies, mainly IgG, cross the placenta to the fetus. Passive artificial immunity is given as ready-made antibodies.

Antigen processing is the step in which a foreign protein is taken up, cut into peptides, and displayed on major histocompatibility complex (MHC) molecules so that T cells can recognise it. Antigen-presenting cells carry out this step. Macrophages engulf pathogens and present peptides on MHC class II molecules. B-cells bind antigen through their surface immunoglobulin, internalise it, and present its peptides on MHC class II as well. Red blood cells are anucleate and carry no MHC, so they cannot process antigen, and platelets act in haemostasis rather than immunity.

Autoimmune disease occurs when the immune system attacks the body's own antigens. Each disease is linked to a characteristic self-antigen. In rheumatoid arthritis the target is the connective tissue of the joints. In systemic lupus erythematosus the autoantibodies are directed against DNA and other nuclear components. In myasthenia gravis antibodies block or destroy acetylcholine receptors at the neuromuscular junction. In multiple sclerosis the attack falls on myelin of the central nervous system.

Parasitic helminths occupy specific niches in the host, and this site of residence is a standard point of comparison. The adult roundworm Ascaris lumbricoides lives in the lumen of the small intestine. Enterobius vermicularis, the pinworm, lives in the caecum and vermiform appendix, and gravid females migrate to the perianal region. Dracunculus medinensis, the guinea worm, migrates through subcutaneous tissue, usually of the lower limbs. Adult Wuchereria bancrofti lives in lymphatic vessels and lymph nodes, which produces lymphatic filariasis.

Vector-borne parasites

A vector is an arthropod or other animal that transmits a parasite from one host to another. A biological vector is one in which the parasite undergoes development or multiplication, such as the sexual cycle of malaria parasites inside the mosquito. A mechanical vector only carries the parasite passively. Mosquitoes are the vectors of several important human parasites.

Filariasis is a disease caused by filarial nematodes that live in the lymphatic system. Wuchereria bancrofti and Brugia malayi are the two principal causative agents of lymphatic filariasis, and both are transmitted by mosquitoes. Filariae are nematodes, not protozoa. Visceral leishmaniasis and African trypanosomiasis are caused by protozoa, so they do not count as filariasis.

Several mosquito-borne parasites should be memorised with their vectors. Plasmodium vivax, the agent of benign tertian malaria, is transmitted by Anopheles mosquitoes, in which its sexual phase occurs. Wuchereria bancrofti is transmitted by Culex, Anopheles and Aedes mosquitoes. By contrast, Leishmania donovani is transmitted by sandflies, and Trichuris trichiura is a soil-transmitted nematode that spreads by the faecal-oral route with no arthropod vector.