PARASITOLOGY

n 7.2 INFLAMMATION CAUSED BY PARASITES

In tissues damaged by physical injury or pathogen invasion the tissue mast cells degranu-

late, releasing histamine and serotonin, vasoactive amines. Those compounds cause

relaxation of the smooth muscles of the capillaries and arterioles and affect the perme-

ability of membranes of local cells. The vasoactive amines loosen the adhesions between

adjacent cell membranes, forming gaps between adjacent epithelial cells. Blood plasma

now leaks into the surrounding tissue and leukocytes migrate from the lumen of the

vessels into the tissues (a process known as diapedesis, see Fig. 7.1). There is an increase

in blood flow to the infected site or injury. This is followed by a local temperature

increase due to the secretions of pyrogens secreted by leukocytes (mainly neutrophils)

and redness due to leakage of body fluids.

The leakage of fluids and cells is drawn by chemotactic and cytokine stimuli to the

site of parasite invasion. The local cells are activated by the presence of the pathogen and

secrete various chemotactic stimulants to attract leukocytes into the area. Circulating

leukocytes are attracted to the site of infection by the pathogen-activated cell adhesion

molecules (CAM) on the membranes of epithelial cells. On the external cell membranes

of circulating leukocytes are ligands — molecules which when activated bind to the

appropriate cell adhesion molecules (see section 5.7). Tissue damage and movement

through the tissue disrupts local nerve endings. The subsequent neurological response is

a contraction of local smooth blood vessels and a temporary alteration in the blood flow.

PARASITOLOGY

• Figure 7.2During
  an infectious state the
  CAM molecules on the
  endothelial cells of the
  nearest blood vessels
  are activated. The ligands
  on the leukocytes
  become receptive and
  the leukocytes then
  accumulate by adhering to
  the CAM molecules. The
  walls of the endothelial
  cells are affected by
  various vaso-active factors
  released from locally
  activated mast cells and
  this then permits the
  leukocytes to migrate from
  the blood vessels into the
  tissues to the site of the
  infection.

Lumen

Migrating

white

blood cells

Endothelial cells on the wall of a capillary blood vessels

Ligands

CAM molecules

• Figure 7.1The
  leukocytes circulate round
  the body in the blood
  vessels. The endothelial
  cells on the walls of blood
  vessels have various cell
  adhesion molecules (CAM)
  on their membranes.
  On the surface of the
  leukocytes are CAM
  receptor molecules known
  as ligands.

CAM molecules

Migration of white blood cells through endothelium

Lumen of

blood vessel