Host Response to Parasites

Eosinophils are granulocytes specifically recruited and activated against multicellular parasites, particularly during tissue-invasive or migratory phases of helminth infection. They contain cytotoxic granule proteins, including major basic protein, eosinophil cationic protein and eosinophil peroxidase. These are released onto the surface of extracellular targets too large for phagocytosis.

This mechanism, extracellular degranulation onto an antibody-coated target, is fundamentally different from the intracellular killing that neutrophils and macrophages use against bacteria. It reflects the different scale of challenge that helminths present. A multicellular larva or adult worm cannot be engulfed by a single phagocyte, so the host has evolved a cell type specialised in externalised, surface-directed cytotoxicity.

Eosinophil production and recruitment is driven substantially by interleukin-5, part of the broader Th2 cytokine programme. Eosinophil counts in peripheral blood are used clinically as an indirect marker of active tissue-invasive helminth infection.

The correlation is imperfect and can be blunted by corticosteroids or other immunosuppression. This is of specific importance in Strongyloides hyperinfection, where eosinophilia is often paradoxically reduced or absent.

Immunoglobulin E is the antibody isotype most closely associated with anti-helminth immunity, produced by B cells under the influence of the same Th2 cytokine environment that drives eosinophil production. IgE binds to Fc receptors on mast cells and basophils, sensitising them to subsequent antigen exposure.

Parasite-specific IgE contributes to host defence in two ways. It coats parasite surfaces to facilitate eosinophil-mediated killing (antibody-dependent cellular cytotoxicity). It also triggers mast cell degranulation on repeat antigen exposure, producing local inflammatory changes that can help expel luminal parasites.

The same IgE-mediated mechanism that assists parasite defence is directly responsible for many of the hypersensitivity phenomena seen in parasitic disease. These include cercarial dermatitis, Calabar swellings in loiasis, and the acute urticarial and anaphylactoid reactions that can accompany antigen release from ruptured hydatid cysts.

The same immune pathway can therefore be protective in one context and directly pathological in another.

Mast cells, sensitised by parasite-specific IgE bound to their surface Fc receptors, degranulate on subsequent contact with the relevant antigen, releasing histamine and other mediators that increase vascular permeability, recruit further inflammatory cells, and, in the intestinal setting, increase mucus secretion and smooth muscle contractility.

In intestinal helminth infection, this mast cell-driven change in gut physiology, sometimes described as the "weep and sweep" response, is thought to contribute directly to physical expulsion of luminal worms by increasing fluid secretion and peristaltic activity, mechanically dislodging parasites from the mucosal surface.

Mast cell activation is also the basis of several of the acute clinical presentations including the local hypersensitivity swelling produced by a migrating Loa loa adult worm and the systemic reaction associated with cercarial penetration in some individuals.

Macrophages play a dual and organism-dependent role in parasitic infection. Against protozoa capable of intracellular survival, activated macrophages are the principal effector cell. They are driven by interferon-gamma in a Th1-polarised response and use enhanced oxidative and nitric oxide-mediated killing mechanisms to destroy intracellular organisms such as Leishmania amastigotes and Toxoplasma tachyzoites.

Paradoxically, macrophages are also the specific host cell exploited by several of these same organisms for their own survival and replication. The outcome of infection depends on whether the macrophage is adequately activated, favouring parasite killing, or remains in a less activated state that the parasite can tolerate or actively promote.

In helminth infection, macrophages contribute to granuloma formation, walling off larvae or eggs too large to be destroyed outright. In an alternatively activated (Th2-associated) state, they contribute to tissue repair and fibrosis around chronic parasitic lesions. This role is distinct from their classically activated, Th1-driven antimicrobial function against intracellular protozoa.

Granulomas are organised collections of macrophages, lymphocytes and other immune cells. They form around a parasite or parasite product too large to be cleared by phagocytosis alone, and serve to physically contain the target and limit diffusion of its antigens into surrounding tissue.

This response is central to the pathology of several parasitic diseases. Schistosome egg granulomas drive the periportal fibrosis and portal hypertension of hepatic schistosomiasis. Toxocara larvae provoke granulomatous reactions wherever they lodge during aberrant tissue migration in visceral and ocular larva migrans.

Granuloma formation illustrates a recurring theme in parasitic immunology. The host response that limits direct antigen toxicity is frequently the same response responsible for the chronic organ damage that defines long-term parasitic disease. The granuloma is therefore simultaneously protective and pathological, depending on the timeframe considered.

A Th1-polarised immune response is characterised by interferon-gamma production, activation of macrophages toward an enhanced intracellular killing phenotype, and support for cell-mediated cytotoxic mechanisms. It is generally the effective, protective response against intracellular protozoa such as Leishmania, Toxoplasma and, to a significant extent, Trypanosoma cruzi.

An adequate Th1 response allows most immunocompetent hosts to control Leishmania infection as self-limited cutaneous disease rather than progressive dissemination.

Its impairment, whether from HIV, other T-cell dysfunction or age, is central to why visceral leishmaniasis and toxoplasmic reactivation behave as opportunistic diseases in immunocompromised populations.

A Th2-polarised immune response is characterised by interleukin-4, interleukin-5 and interleukin-13 production. These drive IgE class switching in B cells, eosinophil production and recruitment, mast cell activation and increased mucus secretion. It is the dominant and generally protective response pattern against multicellular helminths.

The Th1/Th2 balance is not simply additive but reciprocally regulatory, with a strong response of one type tending to suppress the other. This has direct clinical relevance in co-infection: a Th2-skewed immune state from chronic helminth infection can impair Th1-dependent control of coexisting intracellular pathogens.

The phenomenon is of ongoing research interest in regions where helminth infection and diseases such as tuberculosis or HIV are both highly prevalent.

Parasites have evolved a wide range of specific mechanisms to blunt, redirect or escape these host responses. Recognising the evasion strategy used by a given organism explains much of its characteristic clinical behaviour.

Antigenic variation is most highly developed in African trypanosomes. The parasite periodically changes its surface glycoprotein coat, staying ahead of an antibody response raised against previous variants. This produces the characteristic relapsing parasitaemia and fever pattern of sleeping sickness.

Intracellular sequestration is used by Toxoplasma, Leishmania and Trypanosoma cruzi during its amastigote phase. It physically shields the parasite from circulating antibody and complement. The host must rely on cell-mediated mechanisms capable of recognising and destroying infected cells rather than the free organism itself.

Antigen mimicry and surface antigen acquisition are used by adult schistosomes. The parasite displays host-derived or host-like molecules on its surface, reducing immune recognition of what is, immunologically, a large and otherwise highly exposed extracellular organism living for years within the bloodstream.

Anatomical sequestration is used by Plasmodium falciparum, through cytoadherence of infected erythrocytes to deep vascular endothelium, and by Onchocerca volvulus adults within fibrous subcutaneous nodules. It physically removes the parasite or its mature life cycle stages from ready immune surveillance in the circulating blood.

Some organisms suppress or misdirect the host immune response itself, seen to varying degrees across many chronic parasitic infections. Examples are the impaired Th1 response associated with progressive visceral leishmaniasis and the broader Th2 skewing associated with chronic helminth infection. This allows the organism to actively favour an immune environment less capable of clearing it, rather than relying solely on passive concealment.