Biology 12

Chapter 4 Homeostatic Mechanisms • MHR 125

4.4 Homeostasis and the Immune System

Another homeostatic mechanism that we depend
on for existence is the immune system. The
immune system comprises a variety of white blood
cells and proteins that attack foreign invaders such
as bacteria and viruses. This system recognizes and
destroys damaged cells and irregular growths such
as tumours. The human immune system must
distinguish harmless from harmful organisms and
must deal with both in efficient ways that pose no
risk to human health.
The air we breathe, the water we drink, and the
food we eat are all inhabited by millions of unseen
organisms. For many of these organisms, the human
body is an ideal habitat. The cells of the body are
kept moist, warm, and well fed by our homeostatic
systems. Here many bacteria, viruses, and parasites
are able to reproduce at incredible rates and spread
throughout the body or beyond in search of new,
favourable habitats. Over millions of years, many
of these organisms have become adapted to allow
them to penetrate the body’s defences or live in
particular places in or on the body. Many of these
organisms cause little or no damage to us. In fact,
it is estimated that the human body is host to more
than 10 000 times more foreign cells (usually
bacteria) than human cells. In some cases these
close associations are mutualistic — both the host
human and the micro-organism benefit. For
example, the inner walls of the large intestine are
coated with a beneficial strain, or form, of the
bacteria Escherichia coli(E. coli) that helps its
human host to absorb nutrients (see Figure 4.16).
Other invaders, including strains of the same
species, can prove dangerous. Before the 1950’s the
most common cause of death in North America was
communicable disease. The use of antibiotics and
vaccines has diminished this risk, but the reprieve

may prove temporary, as many bacteria and viruses

develop resistance.

Figure 4.16Some strains of the Escherichia coli (E. coli)

bacteria are beneficial to humans; others can be harmful.

Non-specific Defences and

Specific Immunity

The human body survives by means of various

defences. These defences can be divided into two

groups — those that provide non-specific defence

and those that provide specific immunity. Immunity

is the ability to resist a disease after being exposed

to it in the past. Non-specific defencesguard against

a wide variety of pathogens(disease-causing agents).

Their effectiveness does not depend on prior

exposure to the pathogen. The first layer of defence

is the skin. Although thousands of bacteria, fungi,

and other pathogens can be found on the skin, it is

EXPECTATIONS

Describe the mammalian immunological response to a viral or bacterial

infection.

Describe the role of the immune system in maintaining homeostasis in

the body.

Predict the impact of environmental factors such as allergens on

homeostasis within an organism.

Compile and display, based on data you collect on food allergies from an

investigation, information on the impact of allergies on the homeostasis

of those affected.