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ANATOMY & PHYSIOLOGY II
THE BODY SYSTEMS
Anatomy & Physiology II – The Body Systems
©Michelle Cochrane 2014
All rights reserved. This material is subject to copyright and may not be
reprinted or reproduced in any manner without prior consent from the author.
Reproduced by Om Shanti College with permission from the author for
educational purposes only.
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LESSON 5 – IMMUNE SYSTEM
5.1 Overview of the Immune System
5.1.1 Pathogens
5.1.2 Types of Defence
5.2 Innate Defence – External
5.2.1 Skin
5.2.2 Mucous Membranes
5.3 Innate Defence - Internal
5.3.1 Phagocytes
5.3.2 Natural Killer Cells
5.3.3 Inflammation
5.3.4 Fever
5.4 Adaptive Defence
5.4.1 Antigens and Antibodies
5.4.2 T Cells
5.4.3 B Cells
5.4.4 Vaccination
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5.1 OVERVIEW OF THE IMMUNE SYSTEM
Every hour of everyday our body comes under attack from the outside. Whether it be
bacteria, viruses, fungi, parasites or environmental toxins, they all pose a risk to our health.
Our immune system is the defence mechanism that our body has developed to deal with all
of these threats to keep us healthy.
The immune system also keeps us healthy by looking out for abnormal cells such as cancer
cells.
Unfortunately, sometimes it makes mistakes and launches a defence against substances
that are not really harmful. In the case of allergies, this is where normally harmless
substances are treated as toxic. In auto-immune disease, the body turns on its own cells
and tissues.
5.1.1 PATHOGENS
Pathogens are substances that can cause harm to the body. These can include:
 Viruses
 Bacteria
 Fungi
 Parasites
 Environmental toxins
 Other poisons
Pathogens can be divided into two main categories:
 Antigens – these are substances which are genuinely harmful to the body and
require an immune response e.g. bacteria, viruses
 Allergens – substances which are normally harmless, but the body has an abnormal
immune response to them e.g. peanuts, shellfish, latex
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5.1.2 TYPES OF DEFENCE
There are two basic types of defence.
The innate defence is our body’s immediate response to the threat. It happens within
minutes of coming into contact with the pathogen. This is a broad brush approach,
attacking everything that poses a potential threat. The innate defence can be further
divided into two parts. The external innate response consists of our skin and mucous
membranes and is sometimes called the first line of defence. The second line of defence
happens inside the body. This internal innate defence involves a response by our immune
cells as well as types of reaction like inflammation and fever.
The adaptive response is slower to respond. It is more targeted in its approach, attacking
specific pathogens that it recognises. This is sometimes called the third line of response.
LINES OF DEFENCE
ADAPTIVE
DEFENCE
INNATE DEFENCE
1ST LINE
2ND LINE
3RD LINE
(EXTERNAL)
(INTERNAL)
(IMMUNITY)
We will have a more in depth look at each of these in turn in the following sections.
5.2 INNATE DEFENCE – EXTERNAL
The external innate defence system is made up of physical barriers – that is our skin and
mucous membranes.
5.2.1 SKIN
The surface of our skin is made from dead keratin cells which provide a water resistant
physical barrier to substances entering the body. It is resistant to mild acids and alkalis as
well as bacteria and toxins.
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Secretions from the sebaceous glands of the skin provide a slightly acidic film over the skin
known as the acid mantle which helps to destroy bacteria on the surface of the skin.
Skin cells are constantly being shed which also helps with the removal of microbes form the
skin.
5.2.2 MUCOUS MEMBRANES
Our mucous membranes are also made of epithelial tissue and line all the internal parts of
the body which have an opening to the exterior. This includes the respiratory and digestive
systems.
Mucous membranes are more permeable than skin but they also have a range of other
defences to guard against pathogens.
 Mucous is the fluid which is secreted by the membranes. It protects the linings of
the digestive and respiratory systems. It helps by trapping microbes that try to
enter the body through the mouth and nose before they get to either the stomach
or lungs.
 Nasal hairs in the nose trap dust and other substances that enter the nose when we
breathe in and stop them before they enter the throat and lungs.
 Cilia are similar to hairs, but they lie deeper inside the respiratory and digestive
passages. They help to move any substances that have been trapped in the mucous
back out of the system again.
 Gastric acid is secreted by the lining of the stomach. It is a very powerful acid which
destroys bacteria and toxins.
 Tears in the eyes wash out debris and microbes that enter the eye.
 Saliva destroys bacteria and microbes that enter the mouth.
5.3 INNATE DEFENCE – INTERNAL
Sometimes pathogens manage to make it past the first line of defence and enter the body.
That is when our internal defence mechanisms come into play.
The innate internal defences are the first to react, often within minutes of detecting the
harmful invader. This quick reaction can often save us from systemic illness.
Two types of cells are involved in destroying pathogens that enter our body – phagocytes
and natural killer cells. As well as this, the inflammatory process helps to deal with
pathogens where they are found.
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5.3.1 PHAGOCYTES
Phagocytes destroy bacteria and
other pathogens by first
engulfing them and then
breaking them down.
There are two types of phagocytes which are involved in our innate defences – neutrophils
and macrophages.
Neutrophils
Neutrophils are the most common type of white blood cell, accounting for 60-70% of the
total number of WBCs.
Neutrophils play an important role in our immune system. They are primarily responsible
for the destruction of bacteria and fungi which they do by phagocytosis.
Neutrophils are the main component of pus. After they phagocytise the bacteria, they no
longer have a purpose and they die. It is these dead cells which give pus its whitish yellow
colour.
Macrophages
Macrophages start off life as monocytes, another type of white blood cell. In the event that
pathogens enter the body, monocytes leave the blood stream and migrate to the place of
infection where they develop into macrophages.
Macrophages then phagocytise bacteria, dead cells and other debris. They also increase the
inflammatory response.
5.3.2 NATURAL KILLER CELLS
Natural killer cells (NK cells) are a type of lymphocyte that circulate in both the blood and
the lymph. As well as being in the circulation, they are also found in the spleen and lymph
nodes.
NK cells operate by targeting and destroying cells that are infected by bacteria viruses as
well as abnormal cells such as cancer cells.
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NK cells do not discriminate and will attack any cells that are either infected or abnormal.
NK cells do not operate by phagocytosis. Rather, they act by releasing chemicals which then
trigger the cell to kill itself – a process called apoptosis.
5.3.3 INFLAMMATION
The inflammatory response is the body’s reaction to injury resulting from :
 Physical trauma
 Intense heat
 Chemical irritation
 Pathogens – bacteria, viruses, fungi, parasites
There are five key signs and symptoms of inflammation:
 Redness
 Heat
 Pain
 Swelling
 Loss of function (depending on location and extent of inflammation)
The purposes of inflammation are to:
 Prevent the spread of pathogens and toxins to surrounding tissues
 Dispose of pathogens and cell debris
 Alert the adaptive immune system
 Initiate the repair process
There are 3 stages to the inflammation process:
1. Vascular stage
2. Cellular stage
3. Repair stage
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Vascular Stage
Blood vessels dilate allowing more blood flow to the affected area. As the capillaries
expand, this also makes them more permeable – as they are stretched, so the gaps in the
membrane become larger. This enables fluid, lymphocytes and various chemicals to pass
from the capillaries to the surrounding tissues. This is what is responsible for causing the
redness, heat and swelling associated with inflammation.
Cellular phase
Chemicals released into the injured area during the vascular stage attract immune cells
including neutrophils and macrophages which destroy pathogens and damaged tissues by
phagocytosis.
The complement system, a series of proteins found in the blood, also increases
inflammation and phagocytosis as well as promoting other immune responses.
Repair stage
As pathogens and dead or injured tissues are removed, the repair process is able to begin.
This can happen in one of two ways. The damaged cells can be replaced with new cells of
the same type, often with no sign of the original injury. However they are sometimes
replaced with connective tissue which results in the formation of scars. Scars can occur
both internally and externally to the body.
5.3.4 FEVER
Fever is a separate process to inflammation and they should not be confused.
The inflammation process is described above. The heat that results from inflammation is
localised, limited to the site of the injury and the surrounding area. The heat results from
the increased blood flow to the area.
Fever is very different. Fever is defined as an abnormally high body temperature that is not
related to the external environment or physical activity. It occurs when the hypothalamus
in the brain changes the temperature set point for the body.
Fever is usually caused by a bacterial or viral infection although there can be other causes
including cancer or reaction to surgery or trauma.
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It is not known why this occurs but is thought to help in the production of T cells and
antibodies. The higher temperature may also help to kill some microbes. It also increases
the metabolic rate which can speed up the process of tissue repair.
5.4 ADAPTIVE DEFENCE
The adaptive defence system is the third line of defence, and has developed to deal with
infection by pathogens which are able to breach our innate defences.
Adaptive defences target specific pathogens which they recognise as harmful. These are
known as antigens and are pathogens that the body has seen before and has developed a
specific defence against them.
5.4.1 ANTIGENS AND ANTIBODIES
Antigens
Antigens are foreign substances that enter the body and stimulate an immune response.
These can include organisms such as:
 Bacteria
 Viruses
 Fungi
 Parasites
They can also include other substances such as pollen, plant toxins, venom and transplanted
organs.
Antibodies
Antibodies are a type of protein also known as immunoglobulins (immuno = immune,
globulin = protein). Which are formed by B cells and released in response to a specific
antigen.
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There are five different types of antibodies, all with different jobs to do. They are all
involved in the immune system. The two most important types are:
 Immunoglobulin G (IgG) – makes up about 75% of the total antibodies in the blood
and lymph. Enhances phagocytosis and other immune responses.
 immunoglobulin E (IgE) – triggers histamine. Involved in allergic responses.
5.4.2 T CELLS
T cells are a type of lymphocyte that is developed in the bone marrow and then matured in
the thymus. (this is why they are called T cells – T for thymus) Most of this happens during
infancy and early childhood so the thymus loses its function and shrinks after adolescence.
When the immature T cells enter the thymus, they undergo a process of positive and
negative selection which determines whether they are able to survive to maturity
depending on their ability to respond correctly to certain triggers. The T cells that respond
incorrectly are destroyed, with only about 1-2% surviving to become mature T cells. They
are then released into the blood and lymphatic system.
There are several types of T cells, with the important ones being:
 Helper T cells
 Killer T cells
 Memory T cells
Bone
Marrow
Killer T
Cells
Thymus
Memory
Cells
Helper T
Cells
Helper T Cells
Helper T cells help to regulate the immune system. When they recognise an antigen they
secrete chemicals called cytokines which activate the killer T cells, B cells, NK cells and
macrophages which then fight the antigens.
Allergic responses can be triggered by helper T cells incorrectly identifying a substance as an
antigen.
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Killer T Cells
These are also known as cytotoxic T cells. They recognise antigen markers on the surface of
other cells which are infected by viruses or cancer. The killer T cells then release chemicals
which cause the target cell to self-destruct in a process known as apoptosis.
Memory T Cells
Memory T cells remember the pathogen and will recognise it if they encounter it again.
They are then able to mount a much swifter response to fight the pathogen second time
round.
5.4.3 B CELLS
T cells are a type of lymphocyte that is developed in the bone marrow and then migrate to
the spleen and lymph nodes. When activated they release antibodies to fight antibodies.
When activated by the presence of antigens, B cells can develop into one of two types:
 Plasma B cells
 Memory B cells
Plasma B Cells
Most of the B cells will develop into plasma cells when they are activated. These plasma
cells produce antibodies – immunoglobulins (proteins) which are designed to target a
specific antigen. They then release the antibodies into the blood plasma (hence the name
‘plasma cell’).
These antibodies then trigger other immune responses including phagocytes and T cells.
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Memory B Cells
A small amount of B cells do not change into plasma cells but remain as memory B cells.
These cells will then recognise the pathogen again in the case of a repeat infection. They
will migrate throughout the body until they are activated by recognising the pathogen. This
will allow a faster immune response next time round.
5.4.4 VACCINATION
Many infectious diseases, especially bacteria and viruses, can be very serious or even fatal.
Even diseases that we may not think of as particularly harmful – such as measles or
whooping cough – can be fatal in vulnerable people.
Fortunately, the incidence of many of these diseases has significantly declined and some
have been eradicated. A lot of this is thanks to vaccinations. As more people are
vaccinated, the disease is not able to spread and infect new victims so it becomes less
common.
So how do vaccines work? Usually they are administered by injection. There are several
different types of vaccine. They can contain:
 A small amount of a live but weakened virus
 A small amount of a dead bacteria or virus
 A small amount of a modified toxin produced by a bacteria
Administering the vaccine allows the body to develop antibodies to guard against future
infection but without the serious symptoms which would normally accompany infection.
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