126 MHR • Unit 2 Homeostasis
not a hospitable environment for these organisms.
The outer layer of the skin is dry and contains large
amounts of tough, relatively indigestible keratin.
The skin’s oil contains bactericides, and sweat
forms an acidic layer that is inhospitable for
microbial growth. The linings of the respiratory
and digestive pathways also contribute to non-
specific defence. They are covered with thick
mucus that continually traps and sweeps away
potentially dangerous micro-organisms. In
addition, the stomach’s acid kills most of the
micro-organisms in the food we eat.
Non-specific defence also includes the
macrophages, neutrophils, and monocytes that kill
bacteria by the process of phagocytosis, in which
the cell ingests the bacteria to destroy it.
Macrophagesare phagocytic cells found in the
liver, spleen, brain, and lungs; they also circulate
in the bloodstream and interstitial fluid. Neutrophils
and monocytesare white blood cells (leucocytes)
that attack bacteria using phagocytosis. Non-specific
defence also includes natural killer cells, which
are other white blood cells that carry out
phagocytosis. In this case, their targets are body cells
that have become cancerous or infected by viruses.
The specific immune systemincludes a wide
variety of cells that recognize foreign substances
and act to neutralize or destroy them. Over time,
and as a result of the variation in our genetic make-
up, each of us develops an immune system unique
in its capability to deal with a wide variety of
possible infections. We are not all exposed to the
same diseases, and some diseases require a stronger
response than others. The specific immune system
is primarily a function of the lymphocytes of the
circulatory system. Lymphocytesare specialized
white blood cells formed in the bone marrow. They
are divided into two specialized groups, depending
on where they mature. B lymphocytes (B cells)
mature in the bone marrow, while T lymphocytes
(T cells) mature in the thymus gland, which is near
the heart. (Chapter 6 will examine the role of
glands in homeostasis in greater detail.)Cellular Immunity and
Antibody Immunity
B cells and T cells work together to attack invaders.
Cellular immunityis primarily a function of T cells
(see Figure 4.17), while antibody immunityis
performed by B cells. Both systems are controlled
by T cells and are initiated by the action of
macrophages.Figure 4.17In cellular immunity, a macrophage engulfs an
antigen, breaks it down, and places part of the foreign
substance on its own cell surface. The macrophage then
binds to the antigen receptor on helper T cells, activating
the cytotoxic T cells to differentiate and produce identical
clones. Some T cells remain behind in the lymph nodes as
memory T cells. These T cells are able to respond rapidly
to a second attack. Other T cells travel out to the infected
tissue to destroy the pathogen. At the infection site,
cytotoxic T cells may either destroy the pathogen directly
or release chemicals that attract other macrophages to
the site.As mentioned earlier, the immune system
depends on its ability to tell the difference between
“self” (the normal cells of the body) and “non-self”
(abnormal cells or invading organisms). “Non-self”
cells are distinguished by having large molecules
(such as proteins) on their surface. These molecules
are called antigens. Macrophages are able to
recognize these cells as “non-self” and engulf them.
During this process, antigens from the foreign cell
are moved to the surface of the macrophage. T cells
and B cells have specialized receptors on their
surface that match specific antigens.
In cellular immunity, a T cell that has a receptor
for the particular antigen attaches to the macrophageCellular
immunityantigens displayed
on surface of
infected cells
stimulatepathogenmacrophageself–non-self antigen
complex displayed on
macrophage’s surfaceengulfed byresults instimulateshelper
T cellcytotoxic
T cellactive
cytotoxic
T cellsmemory
T cellsproducesstimulates