PARASITOLOGY

PARASITOLOGY

n 5.1 INNATE IMMUNITY

The innate immune system probably came into existence once multicellular organisms

evolved. As soon as the division of labour between cells or specialisation of cells became

operative, phagocytic cells acted aggressively toward non-self material that had managed

to penetrate through the outer covering or ‘skin’. As organisms became more complex

so did protective mechanisms (see Fig. 5.1).

Firstly there is the skin barrier which produces secretions containing bactericides

and fungicides to prevent other organisms from living on it or trying pass through it. Secondly

a whole range of wandering phagocytic cells and natural killer (NK) cells have evolved

to destroy any organism that breaches the skin barrier.

In all animals the innate immune system predates the adaptive immune system and the

reasons for believing that are:

n Innate immune systems are found in all multicellular organisms, whereas adaptive

immune systems are found only in vertebrates.

n Innate immune recognition distinguishes self from non-self perfectly, a condition not

completely met by the adaptive immune response.

n The innate immune system uses receptors that can be traced back to invertebrates, whereas

adaptive immunity appears to use mechanisms that lead to clonally specific antibodies

and T cell antigen receptors (TCRs).

Pathogen

Skin, or

outer protective

layer

Host cell

nA. A multicellular organism

Pathogen

Pathogen Receptor

Regions (PRR)

Pathogen Associated

Molecular Pattern

(PAMPs)

Host cell

nB. The membrane of a non-self cell

• Figure 5.1A. In a
  multicellular organism
  all the host cells have
  identical outer membranes,
  whereas the membrane of
  a pathogen is derived from
  a different genotype and is
  different.
  B. The membrane of a
  non-self cell/organism or
  pathogen is different from
  that of the host cells. The
  host surveillance cells
  recognise this difference.
  On the surface membrane
  of the pathogen are
  molecules known as
  Pathogen Associated
  Molecular Patterns
  (PAMPs). Host surveillance
  cells have on their
  membranes groups of
  molecules known as
  Pathogen Receptor
  Regions (PRR) which can
  detect PAMPs.