The term
chemotaxis refers to the movement of leukocytes
(or cells in general), induced by a chemotactic stimulus. Besides
chemotaxis (stimulated, directed migration), leukocytes also
posses two other types of movement:
random migration
(undirected, spontaneous migration) and
chemokinesis
(stimulated, undirected migration). A chemotactic stimulus is
provided by substances that can either attract or repulse the
cells. Thus, chemotactic cell movement can be either positive or
negative, i.e. the cells may move towards the source of
chemotactic substances (towards an increasing concentration
gradient) or in the opposite direction. The positive movement is
typical for leukocytes. Substances possessing chemotactic activity
are called
chemotactic factors ( chemotaxins,
chemoattractants).
Leukocyte chemotaxis (
leukotaxis) is mainly responsible
for their mobilization at the inflammatory site. Both exogenous
and endogenous chemoattractant participate
in this event (Table 1.11).
Exogenous chemotaxins include bacterial oligopeptides
of the FMLP type, lectins, denutured proteins, some lipids and
lipopolysaccharides. Endogenous chemotaxins are produced by
the host organism and are of humoral (complement fragment
C5a, C5
and Ba, fibrinopeptides,
kallikrein and plasminogen
activator) or cellular type
(from different cells -- LTB
,
PAF, chemotactic cytokines etc.)
Interaction between the chemotactic factor and its corresponding receptor triggers a series of coordinated biochemical events which include changes in the cell transmembrane potential, altered cyclic nucleotide levels and ion flow across the cytoplasmic membrane and increased glucose utilization and oxygen consumption. The composition of membrane phospholipids is altered and arachidonic acid, released by phospholipases, is metabolized into a number of biologically active intermediates and products. Within a few minutes, the leukocyte changes from a round to a triangular shape that is oriented along the direction of chemotactic gradient. Reorganization of cytoskeletal contractile elements, particularly actin microfilaments and microtubular structures, contributes to this shape change. Activation of the contractile cell system not only results in migration but also in other form of movement such as enhanced adherence, spreading, endocytosis and secretion of lysosomal enzymes.