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Infection and immunity
Infection and immunity

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4.4  Innate immunity

All animals, even those with much simpler bodies than our own (e.g. parasitic worms) respond to tissue damage in ways that resemble inflammation in humans. They have cells similar to leukocytes and defensive proteins that flood into areas of tissue damage or infection. These leukocytes and proteins can defend the organism from pathogens because they recognise common patterns of molecules that occur in the structures of many different types of pathogens. These pathogen ‘signature’ molecules are known as PAMPs, or pathogen-associated molecular patterns.

The fact that PAMPs are commonly found in unrelated pathogens means that the leukocytes that recognise them cannot tell one type of pathogen from another. This non-specific immune response against pathogens is so widespread among animals that it is described as innate immunity (‘innate’ means ‘inborn’). Some texts use the alternative term ‘natural’ immunity.

The leukocytes involved in innate immunity are of two general types, each with a different action against pathogens:

  • Cytotoxic [sigh-toh-tox-ik] leukocytes, which simply means ‘cell poisoning’ (the prefix ‘cyto’ denotes a cell). These leukocytes have various methods of attaching to the outside of a pathogen and releasing destructive chemicals onto its surface. Worm larvae, bacteria and protists can all be killed this way.
  • Phagocytic [fag-oh-sit-ik] leukocytes (the prefix ‘phago’ comes from a Greek word meaning ‘to eat’), often abbreviated to phagocytes [fag-oh-sigh-tz]. These leukocytes engulf small pathogens such as bacteria, drawing them into the cytosol where destructive chemicals break them down. This action is termed phagocytosis [fag-oh-sigh-toh-siss].

Figure 4 identifies a phagocyte, red blood cells and two tiny spherical bacteria (Staphylococcus aureus) joined together – this species is often found in ‘strings’ of two or more linked cells. Red blood cells are fragile and some have begun to disintegrate in these culture conditions. Note that red blood cells are also known as erythrocytes [air-rith-roh-sigh-tz].

Figure 4  Red blood cells surround a phagocyte following the chemical trail left by two Staphylococcus aureus bacteria.
  • Viruses are small enough for a leukocyte to engulf if the viruses are floating ‘free’ in the bloodstream or tissue fluids, but once in the body, viruses rapidly become hidden from phagocytic leukocytes because they must infect the host’s own cells in order to make new virus particles. Can you suggest how this protects the virus from the innate immune response?

  • Once inside the host’s cells, they are hidden from the self-tolerant phagocytic leukocytes, which can only recognise and engulf material that originated outside the host’s body. (As you will see later, adaptive immunity has a solution to this problem.)

The anti-pathogen activities of certain specialised proteins are important contributors to the innate immune response. They include proteins that accelerate inflammation, target leukocytes onto pathogens or make host cells resistant to invasion by viruses. Their concentration increases rapidly in the bloodstream during an infection and this rise can be detected in blood tests as a diagnostic sign of infection.

In summary, all animals have innate immunity based on cells similar to human leukocytes and defensive proteins that defend the organism against pathogens ‘in general’ in a non-specific way, i.e. these defences cannot distinguish between one type of pathogen and another. Humans and other warm-blooded animals have an additional defensive capability called adaptive immunity, which differentiates specifically between pathogens, as the next section describes.