n Human immunodeficiency virus infection
The human immunodeficiency virus (HIV) is the cause of a potentially fatal
condition known as acquired immunodeficiency syndrome (AIDS). The virus
belongs to a subgroup of viruses known as ‘slow viruses’. These typically
have a long interval between the initial infection and the onset of serious
symptoms – potentially up to ten years and beyond. The virus affects
the helper T cells (CD4+) cells, and as they replicate it too replicates.
Unfortunately, these compromised cells are unable to respond effectively to
viruses, some bacteria and other threats to health (although the first-line
parts of the immune system can provide some protection), and they do not
activate their target B and T cells, reducing the immune system’s response to
such threats. In addition, the virus can cause the helper T cells to stick
together and render them unusable. Finally, the immune system may recognise
these virus-laden cells as invasive and begin to attack its own helper
T cells. Each of these various processes gradually impairs the efficiency of
the B and T cell systems until they are overwhelmed and the onset of serious
illnesses can occur.
Because helper T cells are so key to the body’s response to a variety of
antigens, repeated exposure to antigens that provoke helper T cells to proliferate
can increase the speed of development of the HIV infection and
its progression to AIDS. When the helper T cell count in the blood falls
below 500/mm3, approximately half of the immune system reserve has been
destroyed. At this point, minor infections such as cold sores and fungal infections
begin to appear. Once the helper T cell count falls below 200/mm3, lifethreatening
opportunistic infections and cancers typically occur. AIDS, the
end-point of HIV disease, occurs when the helper T cell count is less than
200/mm3 or with the presence of potentially life-threatening infections such
as pneumonia or a cancer such as Kaposi’s sarcoma.
The immune system is able to identify cells that are part of the body (‘self’)
and those that are ‘non-self’: antigens, developing cancers, and so on. On
occasion, this process breaks down and the immune system treats cells in the
body as non-self and begins to attack them. This can result in a number of
so-called autoimmune conditions, including diabetes, rheumatoid arthritis
and multiple sclerosis.
Two types of diabetes have been identified. In type 1 diabetes, the body does
not produce sufficient insulin in the islets of Langerhans in the pancreas. Its
onset is frequently triggered by an infection, often by one of the Coxsackie
virus family. This virus expresses a protein similar in structure to an enzyme
involved in the production of insulin, and the immune response to this virus
can also destroy the insulin-producing cells in the pancreas.
a malignant tumour of
the connective tissue,
often associated with
AIDS. The tumours
consist of bluish-red or
purple lesions on the
skin. They often appear
first on the feet or
ankles, thighs, arms,
hands and face.
a group of diseases,
including type 1
disease and rheumatoid
by abnormal functioning
of the immune system
in which it produces
antibodies against its
own tissues – it treats
‘self’ as ‘non-self’.
diabetes (type 1 and 2)
a lifelong disease
marked by high levels of
sugar in the blood and a
failure to transfer this to
organs that need it. It
can be caused by too
little insulin (type 1)
resistance to insulin
(type 2), or both.
gland in which the islets
of Langerhans produce
insulin. Also produces
and secretes digestive
behind the stomach.
220 CHAPTER 8 • THE BODY IN HEALTH AND ILLNESS
Insulin normally attaches itself to glucose molecules in the circulatory
system, permitting them to be taken up by the various body organs that
require glucose to provide them with energy. Without insulin, these glucose
molecules cannot be absorbed, leading to high levels of glucose in the blood,
which the body cannot use. Both the high levels of circulating glucose and
the lack of sugar to vital organs can result in significant problems for people
with diabetes. Symptoms are usually sudden in onset and include increased
thirst and urination, constant hunger, weight loss, blurred vision, and
extreme fatigue. If not diagnosed and treated with insulin, a person with type
1 diabetes can lapse into a life-threatening diabetic coma, also known as
diabetic ketoacidosis. In the longer term, diabetes increases the risk of a
variety of health problems, including impaired circulation, peripheral nerve
damage and coronary heart disease.
A second form of the condition is known as type 2 diabetes. In this condition,
the body produces sufficient insulin (or close to sufficient), but the cells
that take up the glucose–insulin molecules become ‘resistant’ to them and
no longer absorb them. Type 2 diabetes often develops later in life and is
associated with obesity – a person’s chances of developing type 2 diabetes
increases by 4 percent for every pound of excess weight. The symptoms of
type 2 diabetes develop gradually; their onset is not as sudden as in type 1
diabetes, and they may include fatigue or nausea, frequent urination, unusual
thirst, weight loss, blurred vision, frequent infections, and slow healing of
wounds or sores. Some people have no symptoms.
n Rheumatoid arthritis
Rheumatoid arthritis is an autoimmune condition that may be triggered by
viruses in individuals with a genetic tendency for the disease. It is a systemic
disease that affects the entire body and is characterised by inflammation of
the membrane lining of the joints – known as the synovium. It is a chronic
condition, characterised by flare-ups and periods of remission. During flareups,
people with the condition experience significant pain, stiffness, warmth,
redness and swelling. Other features include rheumatoid nodules that form
under the skin in areas that experience external pressure, such as the elbows.
It may also be associated with fatigue, loss of appetite, fever and loss of
energy. The inflamed synovium can invade and damage bone and cartilage.
Inflammatory cells release enzymes that may digest bone and cartilage. This
may result in the joint losing its shape and alignment, causing pain and
restricted movement within the joint.
n Multiple sclerosis
Multiple sclerosis (MS) is a neurological condition involving repeated
episodes of inflammation of nervous tissue in any area of the central nervous
system (brain and spinal cord). These episodes occur when the body’s own
immune cells attack the nervous system. The location of the inflamed areas
varies from person to person and from episode to episode. The inflammation
destroys the covering of the nerve cells (myelin sheath) in that area, leaving
multiple areas of scar tissue (sclerosis). This results in slowing or blocking of
the transmission of nerve impulses in that area, leading to the symptoms of
a chronic autoimmune
inflammation of the
joints and marked
(possibly a virus)
triggers an attack of the
synovium in the joint by
the immune system,
which stimulates an
that can lead to
destruction of the joint.
a disorder of the brain
and spinal cord caused
by progressive damage
to the myelin sheath
covering of nerve cells.
This results in decreased
nerve functioning, which
can lead to a variety of
tremor, pain, tingling,
a substance that
contains both protein
and fat (lipid) and
surrounds all nerves
outside the brain. It acts
as a nerve insulator
and helps in the
transmission of nerve
THE DIGESTIVE TRACT 221
MS. As this may occur in any part of the brain or spinal cord, the symptoms
they cause differ markedly across individuals and include loss of limb function,
loss of bowel and/or bladder control, blindness due to inflammation of
the optic nerve, and cognitive impairment. Muscular spasticity is a common
feature, particularly in the upper limbs, and 95 percent of people with MS
experience debilitating fatigue, which prevents any sustained physical activity
in about 40 percent of people. Between 30 and 50 percent of people with
the condition require walking aids or a wheelchair for mobility.
The course of MS differs across individuals. Onset before the age of 15 is
rare, and 20 percent of those who have MS have a benign form of the disease
in which symptoms show little or no progression after the initial attack. A few
people experience malignant MS, resulting in a swift and relentless decline
and significant disability or even death shortly after disease onset. Onset of
this type of MS is usually after the age of 40. The majority of people have an
episodic condition, with acute flare-ups followed by periods of remission.
Each flare-up is usually followed by a failure to recover to previous levels of
function, resulting in a slowly deteriorating condition. Death is usually due
to complications of MS, including choking, pneumonia and renal failure.
The digestive tract
The digestive tract is the system of organs responsible for the ingestion of
food, absorption of nutrients from that food, and finally the expulsion of
waste products from the body. It comprises the following components:
n small intestine, divided into three parts:
n large intestine, also divided into three parts:
n caecum, to which the appendix is attached
n rectum, terminating in the anus.
Each organ in the system has a different role:
n Mouth: here, food is masticated by chewing, causing the release of
enzymes in the saliva and beginning the process of digestion.
n Oesophagus: this transports food from the mouth to the stomach, compressing
it in the process.
n Stomach: here, food is churned and mixed with acid to decompose it
n Small intestine: this is responsible for mixing the bowel contents with
chemicals to break it into its constituent parts and then absorb them
222 CHAPTER 8 • THE BODY IN HEALTH AND ILLNESS
into the bloodstream for transportation to other organs. Key chemicals
involved in this process include bile, which is made by the liver and stored
in the gallbladder and digests fats, and enzyme-rich juices released from
n The large bowel (colon): this is largely responsible for reabsorption of
water from the bowel contents and expulsion of the unused bowel contents.
Movement between and along these various organs is controlled
by a process known as peristalsis. This involves smooth muscle within the
walls of the organs narrowing and the narrow sections moving slowly
along the length of the organ in a series of waves, pushing the bowel
contents forward with each wave.
Figure 8.5 The large and small intestine and related organs.