IB BIOLOGY UNIT 11: ANIMAL PHYSIOLOGY
11.1.1 PATHOGENESIS & TRANSMISSION OF DISEASES
● A pathogen is an agent that causes disease – either a microorganism such as bacteria, fungi,
parasites, a virus or a prion.
● Pathogens are generally species-specific in that their capacity to cause disease which is known
as pathogenesis is limited to a particular species.
● For example, polio and measles are examples of diseases caused by pathogens that
specifically affect human hosts.
● Certain pathogens may cross the species barrier and be able to infect and cause disease in a
range of hosts.
● For example, diseases from animals that can be transmitted to humans include rabies from
dogs, certain strains of influenza such as bird flu and the bubonic plague from rats.
VIRUSES
● Viruses are metabolically inert and incapable of reproducing independently of a host cell.
● They typically consist of an inner core of nucleic acid surrounded by a protein coat called the capsid.
● Viruses can be either DNA-based (adenoviruses) or RNA-based (retroviruses).
PRIONS
● A prion is an infectious protein that has folded abnormally into a structure capable of causing
disease.
● Prions can cause normally folded proteins to refold into the abnormal form and hence propagate
within a host body.
● Infectious prion proteins have a higher beta-sheet content, making them more resistant to denaturation
and difficult to treat.
BACTERIA
● Bacteria are unicellular prokaryotic cells that can reproduce quickly and compete with host cells for
space and nutrition.
● Most bacteria are relatively harmless and some may even form mutualistic relationships with hosts.
● Bacteria may cause disease by producing toxic compounds or releasing these substances when
destroyed.
FUNGI
● Disease-causing fungi usually attack the body surfaces, including the skin and mucous membranes.
● They can be categorised according to whether they are unicellular (yeasts) or multicellular (moulds).
● Examples of fungal infections include thrush (yeast infection) and athlete’s foot (mould infection).
PARASITES
● A parasite is an organism that grows and feeds on an organism to the detriment of the host’s
survival.
,● Transmission of infectious diseases can occur via a number of distinct mechanisms:
● Direct Contact – the transfer of pathogens via physical association or the exchange of body
fluids
● Contamination – ingestion of pathogens growing on, or in, edible food sources
● Airborne – certain pathogens can be transferred in the air via coughing and sneezing
● Vectors – intermediary organisms that transfer pathogens without developing disease
symptoms themselves
11.1.2 LINES OF DEFENCE
● The immune system can be divided into three basic lines of defense against pathogenic infection:
● The first line of defense against infection are the surface barriers that prevent the entry of
pathogens into the body.
● These surface barriers include intact skin and mucous membranes.
● Both the skin and mucous membranes release chemical secretions which restrict the
growth of microbes on their surfaces.
● If pathogens cannot enter the host body, they cannot disrupt normal physiological
functions and cause disease.
● The second line of defense are the non-specific phagocytes and other internal mechanisms
that comprise innate immunity.
● Macrophages migrate to infection sites and engulf foreign bodies.
● Inflammatory responses increase capillary permeability at infected sites, recruiting
macrophages but leading to localised swelling.
● Fever increases body temperature to activate heat-shock proteins and suppress
microbial growth and propagation.
● The third line of defense are the specific lymphocytes that produce antibodies as part of the
adaptive immune response.
● Each B cell produces a specific antibody, and the body has millions of different B cells
capable of detecting distinct antigens.
● Helper T cells regulate B cell activation, ensuring that antibodies are only
mass-produced at the appropriate times.
11.1.3 SURFACE BARRIERS (1ST LINE OF DEFENCE)
● The first line of defence against infectious disease are the surface barriers that prevent the entry of
pathogens into the body.
● These surface barriers include both the skin and mucous membranes.
SKIN
● Skin protects external structures.
● Skin has two primary layers:
, ● The underneath layer is dermis and is considered to be pretty much 'alive'. It contains sweat
glands, capillaries, sensory receptors, and dermal cells which give structure and strength to
the skin.
● The layer on top of dermis is epidermis. This epidermal layer is constantly being replaced as
the underlying dermal cells die and are moved upwards. This layer of mainly dead cells forms
a good barrier against most pathogens.
● Skin also contains biochemical defence agents which are chemicals and enzymes inhibiting
microbial growth on skin.
MUCOUS MEMBRANES
● Pathogens can also enter the body at a few points such as trachea, nasal passages, urethra, and
vagina that are not covered by the skin.
● These entry points are lined with tissue cells that form a mucous membrane.
● They are consist of thin regions of living surface cells that release fluids such as mucus and saliva
to wash away pathogens.
● Mucous membranes contain biochemical defence agents which secrete chemicals that contain
lysosome which can destroy cell walls of the pathogens and cause cell lysis.
● Mucous membranes may contain cilia to aid in the removal of pathogens, along with physical actions
such as coughing and sneezing.
11.1.4 INNATE IMMUNE SYSTEM (2ND LINE OF DEFENCE)
● The second line of defence against infectious disease is the innate immune system.
● A principle component of this line of defence are phagocytic white blood cells (leukocytes)
that engulf and digest foreign bodies.
● These are also called macrophages.
● Other components of the innate immune system include inflammation, fever and
antimicrobial chemicals.
● The innate immune system has two key properties:
● It does not differentiate between different types of pathogens — non-specific.
● It responds to an infection the same way every time — non-adaptive.
● If a pathogen enters the body for the first time, then the immune response is called the
primary immune response.
● Since it is the first encounter of the body with the pathogen, the immune response takes
at least a week or more to be successful. Thus, it is more common to experience the
symptoms associated with the disease.
● If a pathogen enters the body not for the first time, but have infected the host before, then the
immune response is called the secondary immune response.
● A secondary immune response is both quicker and more intense, and thus, the
symptoms are rarely experienced.
, 11.1.6 INFLAMMATION (2ND LINE OF DEFENCE)
● The inflammatory response is the non-specific way in which the body responds when a
pathogen damages body tissue.
1. When tissue damage occurs, localized mast cells and circulating basophils release
a chemical called histamine.
2. Histamine causes local vasodilation and increases capillary permeability to improve
the recruitment of leukocytes to the region.
3. Damaged cells also release chemotactic factors which attract leukocytes to the site of
infection.
● While inflammation is necessary to allow immune cells access to damaged tissue, there
are unavoidable side effects:
● Increased blood flow causes redness and heat, while increased permeability releases
fluids and causes swelling and tenderness.
11.1.7 FEVER (2ND LINE OF DEFENCE)
● A fever is an abnormally high temperature associated with infection and is triggered by the
release of prostaglandins.
● Fever may help to combat infection by reducing the growth rate of microbes via the
inactivation of microbial enzymes.
● It may also increase metabolic activity in body cells and activate heat shock proteins to
strengthen the immune response.
1. A fever occurs when activated leukocytes release pro-inflammatory chemicals called
cytokines.
2. Cytokines stimulate the anterior hypothalamus to produce prostaglandins, which
lead to an increase in body temperature.
● Up to a certain point a fever may be beneficial, but beyond a tolerable limit it can cause
damage to the body’s own enzymes.
11.1.8 MACROPHAGES & PHAGOCYTOSIS (2ND LINE OF DEFENCE)
● Macrophages are large leukocytes that are able to change their cellular shape to surround an
invading pathogen through phagocytosis.
11.1.1 PATHOGENESIS & TRANSMISSION OF DISEASES
● A pathogen is an agent that causes disease – either a microorganism such as bacteria, fungi,
parasites, a virus or a prion.
● Pathogens are generally species-specific in that their capacity to cause disease which is known
as pathogenesis is limited to a particular species.
● For example, polio and measles are examples of diseases caused by pathogens that
specifically affect human hosts.
● Certain pathogens may cross the species barrier and be able to infect and cause disease in a
range of hosts.
● For example, diseases from animals that can be transmitted to humans include rabies from
dogs, certain strains of influenza such as bird flu and the bubonic plague from rats.
VIRUSES
● Viruses are metabolically inert and incapable of reproducing independently of a host cell.
● They typically consist of an inner core of nucleic acid surrounded by a protein coat called the capsid.
● Viruses can be either DNA-based (adenoviruses) or RNA-based (retroviruses).
PRIONS
● A prion is an infectious protein that has folded abnormally into a structure capable of causing
disease.
● Prions can cause normally folded proteins to refold into the abnormal form and hence propagate
within a host body.
● Infectious prion proteins have a higher beta-sheet content, making them more resistant to denaturation
and difficult to treat.
BACTERIA
● Bacteria are unicellular prokaryotic cells that can reproduce quickly and compete with host cells for
space and nutrition.
● Most bacteria are relatively harmless and some may even form mutualistic relationships with hosts.
● Bacteria may cause disease by producing toxic compounds or releasing these substances when
destroyed.
FUNGI
● Disease-causing fungi usually attack the body surfaces, including the skin and mucous membranes.
● They can be categorised according to whether they are unicellular (yeasts) or multicellular (moulds).
● Examples of fungal infections include thrush (yeast infection) and athlete’s foot (mould infection).
PARASITES
● A parasite is an organism that grows and feeds on an organism to the detriment of the host’s
survival.
,● Transmission of infectious diseases can occur via a number of distinct mechanisms:
● Direct Contact – the transfer of pathogens via physical association or the exchange of body
fluids
● Contamination – ingestion of pathogens growing on, or in, edible food sources
● Airborne – certain pathogens can be transferred in the air via coughing and sneezing
● Vectors – intermediary organisms that transfer pathogens without developing disease
symptoms themselves
11.1.2 LINES OF DEFENCE
● The immune system can be divided into three basic lines of defense against pathogenic infection:
● The first line of defense against infection are the surface barriers that prevent the entry of
pathogens into the body.
● These surface barriers include intact skin and mucous membranes.
● Both the skin and mucous membranes release chemical secretions which restrict the
growth of microbes on their surfaces.
● If pathogens cannot enter the host body, they cannot disrupt normal physiological
functions and cause disease.
● The second line of defense are the non-specific phagocytes and other internal mechanisms
that comprise innate immunity.
● Macrophages migrate to infection sites and engulf foreign bodies.
● Inflammatory responses increase capillary permeability at infected sites, recruiting
macrophages but leading to localised swelling.
● Fever increases body temperature to activate heat-shock proteins and suppress
microbial growth and propagation.
● The third line of defense are the specific lymphocytes that produce antibodies as part of the
adaptive immune response.
● Each B cell produces a specific antibody, and the body has millions of different B cells
capable of detecting distinct antigens.
● Helper T cells regulate B cell activation, ensuring that antibodies are only
mass-produced at the appropriate times.
11.1.3 SURFACE BARRIERS (1ST LINE OF DEFENCE)
● The first line of defence against infectious disease are the surface barriers that prevent the entry of
pathogens into the body.
● These surface barriers include both the skin and mucous membranes.
SKIN
● Skin protects external structures.
● Skin has two primary layers:
, ● The underneath layer is dermis and is considered to be pretty much 'alive'. It contains sweat
glands, capillaries, sensory receptors, and dermal cells which give structure and strength to
the skin.
● The layer on top of dermis is epidermis. This epidermal layer is constantly being replaced as
the underlying dermal cells die and are moved upwards. This layer of mainly dead cells forms
a good barrier against most pathogens.
● Skin also contains biochemical defence agents which are chemicals and enzymes inhibiting
microbial growth on skin.
MUCOUS MEMBRANES
● Pathogens can also enter the body at a few points such as trachea, nasal passages, urethra, and
vagina that are not covered by the skin.
● These entry points are lined with tissue cells that form a mucous membrane.
● They are consist of thin regions of living surface cells that release fluids such as mucus and saliva
to wash away pathogens.
● Mucous membranes contain biochemical defence agents which secrete chemicals that contain
lysosome which can destroy cell walls of the pathogens and cause cell lysis.
● Mucous membranes may contain cilia to aid in the removal of pathogens, along with physical actions
such as coughing and sneezing.
11.1.4 INNATE IMMUNE SYSTEM (2ND LINE OF DEFENCE)
● The second line of defence against infectious disease is the innate immune system.
● A principle component of this line of defence are phagocytic white blood cells (leukocytes)
that engulf and digest foreign bodies.
● These are also called macrophages.
● Other components of the innate immune system include inflammation, fever and
antimicrobial chemicals.
● The innate immune system has two key properties:
● It does not differentiate between different types of pathogens — non-specific.
● It responds to an infection the same way every time — non-adaptive.
● If a pathogen enters the body for the first time, then the immune response is called the
primary immune response.
● Since it is the first encounter of the body with the pathogen, the immune response takes
at least a week or more to be successful. Thus, it is more common to experience the
symptoms associated with the disease.
● If a pathogen enters the body not for the first time, but have infected the host before, then the
immune response is called the secondary immune response.
● A secondary immune response is both quicker and more intense, and thus, the
symptoms are rarely experienced.
, 11.1.6 INFLAMMATION (2ND LINE OF DEFENCE)
● The inflammatory response is the non-specific way in which the body responds when a
pathogen damages body tissue.
1. When tissue damage occurs, localized mast cells and circulating basophils release
a chemical called histamine.
2. Histamine causes local vasodilation and increases capillary permeability to improve
the recruitment of leukocytes to the region.
3. Damaged cells also release chemotactic factors which attract leukocytes to the site of
infection.
● While inflammation is necessary to allow immune cells access to damaged tissue, there
are unavoidable side effects:
● Increased blood flow causes redness and heat, while increased permeability releases
fluids and causes swelling and tenderness.
11.1.7 FEVER (2ND LINE OF DEFENCE)
● A fever is an abnormally high temperature associated with infection and is triggered by the
release of prostaglandins.
● Fever may help to combat infection by reducing the growth rate of microbes via the
inactivation of microbial enzymes.
● It may also increase metabolic activity in body cells and activate heat shock proteins to
strengthen the immune response.
1. A fever occurs when activated leukocytes release pro-inflammatory chemicals called
cytokines.
2. Cytokines stimulate the anterior hypothalamus to produce prostaglandins, which
lead to an increase in body temperature.
● Up to a certain point a fever may be beneficial, but beyond a tolerable limit it can cause
damage to the body’s own enzymes.
11.1.8 MACROPHAGES & PHAGOCYTOSIS (2ND LINE OF DEFENCE)
● Macrophages are large leukocytes that are able to change their cellular shape to surround an
invading pathogen through phagocytosis.
