Blood Functions. Defense mechanisms
Cells of the blood and constituents of the plasma interact
in complex ways to confer immunity to infectious agents,
to resist or destroy invading organisms, to produce
the inflammatory response, and to destroy and remove
foreign materials and dead cells. The leukocytes (white
blood cells, discussed later in this section) have a
primary role in these reactions: granulocytes and monocytes
phagocytize (ingest) bacteria and other organisms (see
video), migrate to sites of infection or inflammation
and to areas containing dead tissue, and participate
in the enzymatic breakdown and removal of cellular debris;
lymphocytes are concerned withthe development of immunity.
Acquired resistance to specific microorganisms is in
part attributable to antibodies, proteins that are formed
in response to the entry into the body of a foreign
substance (antigen). Antibodies that have been induced
by microorganisms not only participate in eliminating
the microbes but also prevent reinfection by the same
organism. Cells and antibodies may cooperate in the
destruction of invading bacteria; the antibody may attach
to the organism, thereby rendering it susceptible to
phagocytosis. Involved in some of these reactions is
complement, a group of protein components of plasma
that participates in certain immunologic reactions.
When certain classes of antibodies bind to microorganisms
and other cells, they trigger the attachment of components
of the complement system to the outer membrane of the
target cell. As they assemble on the cell membrane,
the complement components acquire enzymatic properties.
The activated complement system is thus able to injure
the cell by digesting (lysing) portions of the cell's
protective membrane.
