INTRODUCTION TO IMMUNITY
• An immune system is a system of
• biological structures and
• processes within an organism
• that protects against disease.
• In order to function properly, an
immune system must detect a wide
variety of agents, from viruses to
parasitic worms, and distinguish them
from the organism's own healthy
COUNTER ACTION OF THESE AGENTS
These agents are referred to as
pathogens – an organism or substance
that could cause a disease.
Pathogens can rapidly evolve and adapt
to avoid detection and destruction by
the immune system.
As a result, multiple defence
mechanisms have also evolved to
recognize and neutralize pathogens.
Physical barriers prevent pathogens
such as bacteria and viruses from
entering the organism.
If a pathogen breaches these
barriers, the innate immune system
provides an immediate, but nonspecific response.
Innate immune systems are found in all
plants and animals.
If pathogens successfully evade the
innate response, vertebrates possess a
second layer of protection, the acquired
which is activated by the innate
Here, the immune system adapts its
response during an infection to improve
its recognition of the pathogen.
This improved response is then retained
after the pathogen has been eliminated, in
the form of an immunological memory,
and allows the acquired immune system to
mount faster and stronger attacks each
time this pathogen is encountered.
Differences in the components of the
Innate immune system Acquired immune system
Response is nonspecific
Exposure leads to
Found in nearly all
forms of life
Pathogen and antigen
Lag time between exposure
and maximal response
Cell-mediated and humoral
Exposure leads to
Found only in jawed
2. INNATE IMMUNITY
Innate immune responses are active
immediately upon infection and are the
same whether or not the pathogen has
been encountered previously.
It includes barrier defenses
and internal defenses
BARRIER DEFENSE SYSTEM
Mucus membranes of digestive-,
respiratory- urinary and
oBody secretions: mucus, saliva
(lysozymes), tears, oil gland
secretions, acid in stomach, sweat.
Some cells in mucus membrane
Mucus is a viscous fluid that
enhances defences – trapping
microbes and other foreign
In the trachea, ciliated
epithelial cells sweep mucus
and the trapped microbes
upwards, helping to prevent
infection of the lungs.
Body secretions create an
environment that is
unfavourable for microbes.
Lysozymes in saliva, mucous
secretions, and tears destroy
susceptible bacteria as they
enter the respiratory tract or
openings around eyes.
Acid in stomach kill bacteria
Oils and sweat give human skin
a pH between 3-5, which is acidic
enough to prevent the growth of
If the barrier defences are
damaged and pathogens do enter
the body of an organism, a second
line of defence will be activated.
This defence system is the
internal defence system and is
more sensitive and includes:
phagocytosis and inflammation.
process by which certain living
cells called phagocytes ingest or
engulf other cells or particles.
The phagocyte may be a onecelled organism, such as an
amoeba, or one of the body cells,
such as a leukocyte (white blood
In higher animals phagocytosis
is chiefly a defensive reaction
against infection and invasion of
Different types of phagocytic
Neutrophils engulf and destroy
Macrophages are part of the
lymphatic system and are found
throughout the body
Eosinophils discharge destructive
Dendritic cells stimulate
development of acquired immunity
When injured or infected by
pathogens, signalling molecules
One example of a signalling
molecules is histamine – stored in
These molecules trigger the blood
vessels to dilate and become
This increase local blood supply and
allow more phagocytes and
antimicrobial proteins to enter
Pus, a fluid rich in white blood cells,
dead microbes, and cell debris,
accumulates at the site of inflammation
Fever is a systemic inflammatory
response triggered by pyrogens
released by macrophages, and toxins
MAJOR EVENTS IN A LOCAL
and mast cells
at the injury site
act on nearby
The capillaries dilate and
to enter the tissue.
released by immune cells
digest pathogens and
cell debris at the site,
and the tissue heals
Acquired immunity is a specific immune
response system through which the
body specifically detects and destroys
This immunity protects us against
infectious diseases as the body has
learnt to recognize foreign substances.
It produces a specific reaction to each
infectious agent, eradicating that agent
from the body.
This ability to recognize a pathogen
that has previously elicited an immune
response is the basis for acquiring
immunity to specific diseases.
Hence, we suffer from many diseases,
such as chicken pox, measles etc. only
Thus we can summarize: Acquired
immunity involves 2 main activities:
Destruction of the invaders
Memory of this response
WHICH CELLS ARE INVOLVED IN
ACQUIRED IMMUNITY RESPONSE?
Special leucocytes, called the
lymphocytes are released from the
Some reach the Thymus gland and
mature to form T-lymphocytes (Tcells)
Some become B-lymphocytes (Bcells) and is present in the bone
marrow and lymph nodes)
ACTIVE ACQUIRED IMMUNITY
Active immunity occurs when a
person has already been exposed
to antigens (from pathogens)
And develops a secondary response
against specific pathogens.
People receive vaccinations to
develop a primary response, and
then if they get the pathogens later,
active immunity helps to fight
Antigens are foreign molecules,
found on the surface of pathogens,
each pathogen has a specific antigen.
The immunity system has countless
amounts of B-Lymphocytes.
Each B-lymphocyte is able to
recognize a specific antigen.
The B-lymphocytes then produce
antibodies that will bond to the
This will neutralize or destroy the
HOW B-LYMPHOCYTES WORK
o One type of B-lymphocyte is activated by
a specific antigen on the surface of a
oThe B-lymphocyte form antibodies
The antibodies bond to the antigens
and destroy the foreign body.
In natural passive immunity, antibodies are
passed from a mother to a child.
Antibodies can be transferred through the
placenta, or transmitted through the colostrum.
The antibodies transmitted through the
colostrum and placenta generally only last for
several weeks, which is long enough to allow
the baby to start to build up its own immune
system and to make its own antibodies.
Artificial passive immunity involves the
introduction of antibodies through means such
as injection - VACCINATIONS.
HUMORAL VS. CELL MEDIATED
Humoral immunity - deals with infectious
agents in the blood and body tissues
Cell-mediated immunity - deals with body
cells that have been infected.
In general, the humoral system is managed
by B-cells (with help from T-cells).
The cell-mediated system is managed by Tcells.
All vaccinations work by presenting
a foreign antigen to the immune
system in order to evoke an
immune response, but there are
several ways to do this.
We will look at 4 methods:
1. Using an inactivated vaccine
An inactivated vaccine consists of
virus or bacteria that are grown in
culture and then killed .
Although the virus or bacteria particles
are destroyed and cannot replicate, the
virus capsid proteins or bacterial wall
are intact enough to be recognized
and remembered by the immune
This evokes an immune response.
2. Using an attenuated vaccine
In an attenuated vaccine, live virus
or bacteria with very low virulence are
They will replicate, but locally or very
Which causes an immune response to
3. Virus-like particle vaccines
Virus-like particle vaccines consist of
viral protein(s) derived from the
structural proteins of a virus.
These proteins can self-assemble into
particles that resemble the virus from
which they were derived but lack viral
nucleic acid, meaning that they are not
The human papillomavirus and Hepatitis
B virus vaccines are two virus-like
particle-based vaccines currently in
A subunit vaccine
A subunit vaccine presents an
antigen to the immune system
without introducing viral particles.
One method of production involves
isolation of a specific protein from a
virus or bacterium and administering
this by itself.
Antibiotics are also known as antibacterials.
They are drugs used to treat
infections caused by bacteria.
The first antibiotic was penicillin.
How do antibiotics work?
Although there are a number of
different types of antibiotic they all
work in one of two ways:
A bactericidal antibiotic kills the
bacteria. Penicillin is a bactericidal. A
bactericidal usually either interferes with
the formation of the bacterium's cell
wall or its cell contents.
A bacteriostatic stops bacteria from
If antibiotics are overused or
used incorrectly there is a
chance that the bacteria will
become resistant - the antibiotic
becomes less effective against
that type of bacterium.
Campbell and Reece.2010.Biology.8th ed.Cape Town:Pearson.
J. Williamson and E. Pretorius.2014.Life Sciences Learning
Guide.Johannesburg:University of Johannesburg