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P A R T 3
Drugs acting on the immune system
and tissues of each person are marked for identification
as part of that individual’s genetic code. No two people
have exactly the same code. In humans, the genetic iden
tification code is carried on a chromosome and is called
the
major histocompatibility complex
(MHC). The
MHC produces several proteins called histocompatibil
ity antigens, or human leucocyte antigens (HLAs). These
antigens
(proteins) are located on the cell membrane and
allow the body to recognise cells as being self cells. Cells
that do not have these proteins are identified as foreign
and are targeted for destruction by the body.
Cellular defences
Any foreign pathogen that manages to get past the
barrier defences will encounter the human inflamma
tory and immune systems, or mononuclear phagocyte
system (MPS). Previously called the reticuloendothelial
system, the MPS is composed primarily of leucocytes,
lymphocytes, lymphoid tissues and numerous chemical
mediators.
Stem cells in the bone marrow produce two types of
white blood cells or
leucocytes
: the lymphocytes and the
myelocytes. The
lymphocytes
are the key components of
the immune system and consist of T cells, B cells and
natural killer cells (see later discussion of the immune
response). The
myelocytes
develop into a number of
different cell types that are important in both the basic
inflammatory response and the immune response. Mye
locytes include neutrophils, basophils, eosinophils and
monocytes, or macrophages (Figure 15.1).
Neutrophils
Neutrophils are polymorphonuclear leucocytes that
are capable of moving outside of the blood stream (dia
pedesis) and engulfing and digesting foreign material
(
phagocytosis
). When the body is injured or invaded by
a pathogen, neutrophils are rapidly produced and move
to the site of the insult to attack the foreign substance.
Because neutrophils are able to engulf and digest foreign
material, they are called
phagocytes
. Phagocytes are
able to identify non-self cells by use of the MHC, and
they can engulf these cells or mark them for destruction
by cytotoxic T lymphocytes.
Basophils
Basophils are myelocytic leucocytes that are not capable
of phagocytosis. They contain chemical substances or
mediators that are important for initiating and main
taining an immune or inflammatory response. These
substances include histamine, heparin and other chemi
cals used in the inflammatory response.
Basophils that are fixed and do not circulate are
called
mast cells
. They are found in the respiratory
and GI tracts and in the skin. They release many of the
chemical mediators of the inflammatory and immune
responses when they are stimulated by local irritation.
Eosinophils
Eosinophils are circulating myelocytic leucocytes whose
exact function is not understood. They are often found
at the site of allergic reactions and may be responsible
for removing the proteins and active components of the
immune reaction from the site of an allergic response.
Monocytes/Macrophages
Monocytes or mononuclear phagocytes are also called
macrophages
. They are mature leucocytes that are
capable of phagocytising an antigen. Macrophages help
to remove foreign material from the body, including
pathogens, debris from dead cells and necrotic tissue
from injury sites, so that the body can heal. They can
Bone marrow
stem cells
Lymphocytic
cells
Natural killer cells
B cells
Plasma cells
Cytotoxic T cells
Helper T cells
Suppressor T cells
Antibodies
T cells
Neutrophils
Eosinophils
Basophils
Monocytes/macrophages
Myelocytic
cells
FIGURE 15.1
Types of white blood cells, or
leucocytes, produced by the body.
