University of Juba

School of Veterinary Medicine

Course: SVM - 32 - Immunology Class: 3rd Year

Dr. Joseph Badys

Asst. Professor of Molecular Microbiology

Lecture 1 Date: 5th Oct. 2023

 Introduction

• Immunology is a branch of science that deals

with the body’s resistance to infection or the
altered reactivity of the body following an
exposure to a foreign substance or infection.
 Immunology
o the study of immune system or immunity

o the study of all aspects of host defense against

infection and of adverse consequences of immune
responses.

o the study of the physiological mechanisms which

enable the body to recognize materials as foreign and
to neutralize, metabolize or eliminate them without
injury to the host tissue.
 Cont.
• Immunity means the body’s ability to resist
infection or in other words the state of the
body which learns from the experience of past
infection(s), how to deal more efficiently with
subsequent infections.
 Cont.
• The immune system of an individual is consists
of its cells and molecules responsible for the
immunity.

• Immune Response is the reaction of the body

following an exposure to foreign antigen
resulting in the formation of antibodies and
lymphokines
 Theories of Immunology
• Cellular Immunity Theory
• In 1882 the Russian Zoologist Elie Metchnikoff
(1845-1916) observed that when a rose thorn
is introduced into the larvae of a starfish,
within a few hours it was surrounded by
motile cells and could be observed in the
transparent starfish larvae.
 Cont.
• In 1883 he observed that fungal spores could be
attacked by the blood cells in Daphnia, a
metazoan under microscope (being transparent).

• He extended his investigation to mammalian

leukocytes, showing their ability to engulf micro-
organisms, a process which he termed
phagocytosis.
 Cont.
• Thus he arrived to a polarized view that phagocytosis is
the main defense against infection.

• He defined that the phagocytosis is due to two types of

circulating phagocytes:

• The polymorphonuclear leukocytes (microphage) and

• The larger macrophages

 Cont.
• Many people accepted the cellular theory of
Metchnikoff but some pathologist criticized it
as they observed immunity in absence of cells
and about 50 years later again it became an
area of extensive study.
 Humoral Theory
• After Pasteur discovered that immunity can be
produced by vaccination, it was soon recognized
that substances produced immunity are present
in blood or in body fluid (Humor).
• Fodor in 1886 observed a direct action of
immune serum on anthrax bacilli. George Nuttall
in 1888 observed the bactericidal action of blood
in certain animals.
 Cont.
• In 1889 Hans Buchnar showed that cell free
serum is bactericidal and failed to have that
effect after heating at 550C for one hour. The
heat labile substance causing bactericidal
effect was termed alexin (later named as
cytolysin or complement).
 Cont.
• In 1890 Von Behring and Kitasato
demonstrated the neutralizing antitoxic
activity by serum from immunized animals
with diphtheria or tetanus toxin. This was the
first proof of humoral immunity.

• Thus both theories developed simultaneously

 Immune System
• When an animal is born, it emerges from
mother’s womb to a new external
environment where microorganisms are
present.

• To combat the microorganisms, Immune

system must be developed.
 Cont.
• In some species like mice, gestation period is
short and immune system is not fully
developed at the time of birth but in major
domestic animals, gestation period is long and
immune system is fully developed at birth
although not fully functional.
 Cont.
• Development of immune capability depends
on antigenic stimulation but in very early days
they are protected by passive transfer of
antibody from mother.

• The development of immune system in

mammalian fetus follows a consistent pattern.
 Cont.
• Thymus is the first lymphoid organ (primary lymphoid
organ) to be developed followed by secondary
lymphoid organs.
• Primary lymphoid organs are the organs where
differentiation and maturation of lymphocytes takes
place without antigenic stimulation.
• In secondary lymphoid organs, antigenic stimulation of
B and T lymphocytes results in specific immune
reaction.
 Cont.
• Immunoglobulin containing cells develop soon
after the appearance of spleen and lymph
nodes but antibodies are found in late stage of
fetal life.

• Capability to respond to antigen develops

after the lymphoid organs appear.
 Organs of the Immune system
• Lymphoid organs are the important organs of
the immune system.

• Lymphocytes are the highly dynamic cells and

large number of them recirculating as
individual cells in blood, lymph and tissue
fluids.
 Cont.
• Lymphocytes are the predominant cells in
lymphoid organs. They can recognize, respond
and eliminate antigens.
• Lymphocytes either produce humoral or cell
mediated immune (CMI) response or both.
Lymphoid organs can be classified into two
groups;
• Primary lymphoid organs
• Secondary lymphoid organs
 Primary Lymphoid Organs
• The organs that regulate the production and
differentiation of lymphocytes into
immunocompetent lymphocytes without
antigenic stimulation.

• Primary lymphoid organs are also called

generative organs or central lymphoid organs.
 Cont.
• In primary lymphoid organ, lymphocytes first
express antigenic receptors and mature both
phenotypically and functionally without any
antigenic stimulation.

• Mature lymphocytes fall into two major

populations, T cells and B cells depending on
at which primary organ they were matured
 Cont.
• T cells mature in thymus where as B cells in
Bursa of Fabricius (in birds) or bone marrow
(primates and rodents) or Peyer’s patches (in
ruminants and pigs).

• Examples of primary lymphoid organs: bone

marrow, thymus, Bursa of Fabricius, some
Peyer’s patches and fetal liver.
 Secondary Lymphoid Organs
• Are also called peripheral lymphoid organs.
• In these organs antigens are trapped and immune
response is generated based on foreign antigenic
stimulation.
• These organs are rich in macrophages and
dendritic cells (which trap and process antigen)
and also T and B cells (which mediate immune
response).
 Cont.
• Examples of secondary lymphoid organs:
Tonsil, spleen, lymph nodes, mucosal
associated lymphoid tissues (MALT), gut
associated lymphoid tissues (GALT).
 Introduction
• Infectious disease is a major cause of morbidity
and mortality in avian and mammalian species.
• Individual’s body systems supply the host’s
respiratory, nutritional and sensory needs.
• The immune system is uniquely equipped to
provide defense against microbial or parasitic
infection irrespective of the source or the route of
transmission.
 Cont.
• Many pathogenic and opportunistic
microorganisms cause infections in domestic
animals.
• However, Immune responses are not confined to
infectious agents, it also responds to innocuous
substances such as pollens, foreign proteins and
some therapeutic drugs, that can cause
potentially destructive hypersensitivity reactions
 Cont.
• Immune system also detects neoplastic tissue
changes and, in some instances, it triggers the
elimination of such undesirable mutated cells
or neoplastic cells by immune mechanisms
 Microbial colonization of the Host
• Soon after birth the external surfaces of the
body, extensive portions of the alimentary
tract and regions of the respiratory and
urinary tracts become colonized by bacteria.
 Cont.
• The host and colonizing bacteria live in a
relatively peaceful state of coexistence, with
microorganisms restricted to parts of the body
where they can be tolerated and microbial
invasion of tissues can be prevented by
natural antibacterial defense mechanisms.
 Cont.
• Bacteria that colonize tissues of the body
without producing disease constitute part of
the normal flora
 How to improve animals Environment
• This harmonious relationship between
animals and their environment can be
reinforced by good management systems,
optimal nutrition, adequate floor space and
effective disease control programs.
 Influential factors
• Negative factors that can tilt the balance in favour
of potential or actual pathogens include
overcrowding, uncontrolled environmental
temperature, nutritional imbalances and absence
of a disease control program.
• Even if bacteria, fungi or viruses succeed in
entering the tissues and causing infection,
disease is not an inevitable outcome.
 Determinant factors
• Characteristics of the infectious agent,
environmental influences and the susceptibility
of the infected animal usually determine the
outcome of infection.
• If infection is not quickly eliminated, clinical
disease or subclinical infection is the likely
result.
Comparative aspects of innate and adaptive
immunity
 Immunity
o Is the state of protection from infectious diseases.

Immune system

o A remarkably versatile defense system that has

evolved to protect animals from invading
pathogenic microorganisms and cancer.

o It is able to generate an enormous variety of cells

and molecules capable of specifically recognizing
and eliminating an apparently limitless variety of
foreign invaders.
 The Immune System

Immune
System

Innate Adaptive/Acquired
(Nonspecific ) (Specific)

Cellular Humoral Cell Humoral

Components Components Mediated (Ab)
 The immune system
Overview of the Immune System
 Cont.
o Hence, the immune system poses number of
defense lines against the invaders, these
include;

o First line of defense (Nonspecific)

o Second line of defense (Nonspecific)

o Third line of defense (specific)

 Innate Immunity
o Natural immune system (Innate Immunity)
 Non – specific
 First line of defense
 Repeated exposure - no augmentation
o Components
 Biochemical
 Physical
 Cells
Components of 1st line of defense (nonspecific)
• Skin

• Mucous Membranes and their Secretions

- Stomach acid

- Mucous

- Tears

- Urine
 The Second line of Defense (nonspecific)

• Phagocytic white cells

- neutrophils

- monocytes / macrophages

- eosinophils
 Cont.
• Antimicrobial Proteins

- complement

- cytokines (interleukins)

- interferon
 Cont.
• The Inflammatory Response and Fever

- histamines

- cytokines (interleukins)

- pyrogens
 The third line of defense (specific mechanisms)

o Lymphocytes (Cellular response - a 2 prong

attack on invaders)
- B-cells - antibodies
- T-cells - help stimulate B-cells, other T-
cells directly attack infected cells or are involved
in self-regulation of the immune system.
o Antibodies (Humoral response)
 Distinguishing features of 3rd line of defense
• Specificity - the immune system's ability to
recognize and eliminate particular
microorganisms and foreign molecules called
antigens.
• Diversity - the ability of the immune system to
respond to millions of kinds of invaders, each
recognized by its antigenic markers by a unique
antibody producing lymphocyte.
 Self/Non-Self recognition
• Self/Non-Self recognition, why doesn't the immune
system attack our own cells? The standard explanation
is that the immune system is "trained" during its early
development to recognize the difference between self
and non-self. This is called self-tolerance.

• It is presumed that any lymphocytes with receptors for

molecules present in the body (self) before birth (and
perhaps after) are somehow destroyed
 Memory
• Memory - refers to the immune system's ability to
remember antigens it has encountered and to react
quickly and effectively against them a second time
around (This is known as acquired immunity).
• Acquired immunity may be active; conferred by
recovery from a particular infectious disease; or
passive; transferred from one individual to another;
often obtained through vaccination. In the case of
infants mom's antibodies can pass through the
placental barrier.
 Cells of the Immune System
• Immune responses are mediated by a variety of
cells and the soluble molecules that these cells
secrete
• Although the leukocytes are central to all immune
responses, other cells in the tissues also
participate, by signaling to the lymphocytes and
responding to the cytokines (soluble intercellular
signaling molecules) released by T cells and
macrophages
The principal cells of the immune system and the
mediators they produce
 Lines of immune defense mechanisms
The immune system consists of three layers of defense
• The first line of defense is provided by a set of
mechanical (e.g., skin), chemical (e.g., acidic
environment of stomach), and biologic (e.g.,
commensal microbes) barriers that protect the body.
• The second line of body defense is the innate immune
system
• The third is the adaptive immune system
 Components of Innate Immunity (Second
line of defense)
Cellular
• Mast cells
• Neutrophils
• Macrophages
Humoral
• Complement
• Lysozyme
• Interferon
 Components of Adaptive Immunity (third
line of defense)
Cellular
• T lymphocytes
• B lymphocytes
• Plasma cells
Humoral
• Antibodies
• Cytokines