Eosinophils

Eosinophils are also produced in the bone marrow, from where they migrate to areas such as the medulla oblongata, the spleen and the lymph nodes. Some of them will remain in the circulation, where they can live for 8-12 hours, while in tissue they can survive for 8-12 days. During an infection, eosinophils are attracted to the site of infection by chemotactic factors such as CCL11, CCL24, CCL5 and leukotriene B4.

At the site of infection, eosinophils must be activated by cytokines released by T cells, e.g. interleukin-3 (IL-3) and interleukin-5 (IL-5). When activated, eosinophils degranulate; eosinophils are very destructive - their activation has a lot less to do with mediating inflammation and a lot more to do with the actual business of killing pathogens. Eosinophil granules contain cationic granule proteins such as ECP (eosinophil cationic protein) and MBP (major basic protein), which are highly cytotoxic. These can be very destructive - including to host tissue. MBP is able to stimulate the degranulation of mast cells and basophils and ECP is able to create pores in the cell membranes of pathogens, allowing cytotoxic molecules to enter the cell. As well as these, eosinophils release harmful oxygen species - such as superoxide and peroxide - eicosanoids, numerous cytokines - such as TNFα and IL-8 - and various enzymes and growth factors. In addition to this, they release RNases - which are able to combat viral infection - are important mediators of inflammation and may be involved in antigen presentation.

(a) An image of an eosinophil granulocyte. (Image courtesy "Bobjgalindo" (via Wikipedia))

(b) A schematic representation of an eosinophil. (Image courtesy "A. Rad" (via Wikipedia))

Figure 1.56: The eosinophil