PARASITOLOGY

INTRODUCTION TO HOST RESPONSE

However and wherever a pathogen gains entry into the tissues, it will be attacked
by one of the many phagocytic cells that are normally present. If the pathogen is not
completely eliminated, specialised phagocytic cells known as antigen-presenting cells
(APCs) will convey ‘extracts’ of the pathogen (known as antigen) to the lymphatic
system.
It is in the lymphatic organs such as the thymus spleen and lymph nodes where the
antigen ‘reacts’ with the lymphocytes, which is the beginning of the adaptive immune
response (see Fig. 5.2).
The problem with recognising pathogens in general is their molecular variability and
in some instances their high mutational rate which alters their surface structure.
However, pathogenic helminth and protozoan parasites tend to be less variable than
parasitic micro-organisms and viruses. They seem to be able to resist the initial innate
immune response and survive for considerably longer periods within the host. If that occurs
then the infection becomes chronic and the immune reaction develops into an adaptive
immune response.
The mechanisms that come into operation, that is the recognition of the parasite by
the phagocytic cells and subsequent antigen presentation to the lymphocytes, are
thought to have their origins in the primitive systems that still may operate in certain inver-
tebrates. If the current mammalian innate immune system is derived from the original
phagocytic cells which recognise molecular structures of pathogen membranes, this then
implies that there must be a finite number of these molecular patterns. So the surveil-
lance cells of vertebrates, having evolved from the invertebrates, can probably recognise
the molecular patterns associated with the external membranes of most pathogens.
These considerations help the innate immune system to recognise and attempt to
destroy the target (the pathogen).

Pathogen

Skin

Macrophage

Antibodies and effector cells

released into body tissues.

Spleen and lymph nodes

Thymus T cells

T cell precursors

Bone marrow

B cell precursors

• Figure 5.2Once a
  pathogen enters the body
  through the skin (or
  through the cells lining
  the alimentary canal or
  the lungs) a phagocytic
  surveillance cell (eg
  macrophage or dendritic
  cell) engulfs the pathogen.
  The macrophage then
  functions as an antigen-
  presenting cell (APC).
  The APC migrates to the
  lymphatic organs to
  present the antigen to
  T cells and thus start
  an adaptive immune
  response.