PARASITOLOGY

PARASITOLOGY

Eosinophils release activators of hypersensitivity and are phagocytic and can engulf immune

complexes and attack weakened parasites. T cells activated by parasitic helminth secre-

tions secrete cytokines some of which stimulate eosinophils to attach to weakened para-

sites. The eosinophils then discharge their granules onto the parasite tegument.

The antibodies IgG, IgE and IgA attach to activated eosinophil Fc receptors with the

Fab end of the antibody molecules attaching to the pathogen. The antibody forms a

‘bridge’ between the cell and the pathogen and this leads to antibody-dependent cell-

mediated cytotoxicity (ADCC). Eosinophils react to histamine and other pharmacolo-

gically active substances.

What then is the function of eosinophilia? The results of various experiments de-

scribed below may provide some insight as to the role of eosinophils in parasitic infections.

Angiostrongylus canonensisis a nematode for which the mouse is a permissive host but

the rat is not. In rats the parasites migrate and eventually die. In rats there is a pro-

nounced eosinophilia once the nematode begins to weaken. The parasite survives in

mice and the increase in the eosinophil population is far less or almost negligible.

In humans suffering with onchocerciasis during the early stages of the infection, there

is no noticeable eosinophilia while the adult worms are still migrating. Once the adult

worms nodulate, which might be a sign of ageing, there is an increase in the local

eosinophil population.

There is a pentastomid that lives in rattlesnakes and rats. In the rat the parasite under-

goes one or two moults. Eosinophils have been observed digesting the dead casts.

All these results suggest that either eosinophils are active against ageing or damaged

or dying parasites but not against a ‘healthy’ living parasite, or a healthy living parasite

somehow inhibits the eosinophils.

5.6.1. 3Basophils

Basophils (size 10–14 μm) are granulocytes with a round or ring-shaped nucleus and

cytoplasm containing large basophilic granules that stain blue with basic (aniline)

• Figure 5.8A basic
  (pluripotential) stem cell
  differentiates within the
  bone marrow into either
  a myeloid or a lymphoid
  stem cell. All the
  lymphocyte phenotypes
  arise from the lymphoid
  stem cells and all the
  other leukocytes originate
  from the myeloid stem
  cells.

Pluripotential stem cell

Bone marrow

Lymphoid stem cell

T cell precursor

Thymus

Spleen

B cell precursor

Lymph node

Mature T cell

Platelet

Erythrocyte

Neutrophil

Eosinophil

Basophil

Monocyte

Myeloid stem cells