University of Juba

School of Veterinary Medicine

Course: -32 - Immunology Class: 3rd Year

Innate Immunity

Lecture 3 Date: 12th Oct. 2023

 Classification
The discrimination between self and non-self,
and the subsequent destruction and removal of
foreign material, is accomplished by two arms of
immune system;
• the innate (or “natural”) immune system.
• the adaptive (or “acquired”), specific immune
system.
 Innate (or natural) immunity
a. Nonspecific
• - Species
• - Racial
• - Individual
b. Specific
• - Species
• - Racial
• - Individual
 Acquired (or adaptive) immunity
a. Active

• Natural

• Artificial

b. Passive

• Natural

• Artificial
 Innate Immunity
Innate immunity may be classified as

• Individual immunity

• Racial immunity

• Species immunity
 Individual immunity
• Individual immunity denotes resistance to infection,
which varies within different individuals in the same
race and species and is genetically determined.
• For example, if one homozygous twin develops
tuberculosis, there is a very high possibility that the
other twin will also develop tuberculosis. But in
heterozygous twins, there is a very low possibility of
the other twin suffering from tuberculosis
 Racial immunity
• Racial immunity denotes a difference in
susceptibility or resistance to infection among
different races within a same species.
• For example, races with sickle cell anemia
prevalent in Mediterranean coast are immune
to infection caused by malaria parasite
Plasmodium falciparum.
 Cont.
• This is due to a genetic abnormality of
erythrocytes, resulting in sickle shaped
erythrocytes that prevent parasitization by P.
falciparum.

• Similarly, individuals with a hereditary deficiency

of glucose-6-phosphatase dehydrogenase are
also less susceptible to infection by P. falciparum.
 Species immunity
• Species immunity denotes a total or relative
resistance to a pathogen shown by all members
of a particular species.
• For example, chickens are resistant to Bacillus
anthracis, rats are resistant to Corynebacterium
diphtheriae, whereas humans are susceptible to
these bacteria. The exact reason for such type of
immunity is not known
 Factors influencing innate immunity
The factors that may influence innate immunity
of the host include age and nutritional status of
the host;

• Age

• Nutritional status

• Hormonal levels
 Age
• Extremes of age make an individual highly
susceptible to various infections. This is
explained in part by the immature immune
system in very young children and waning
immunity in older individuals.
 Cont.
• The fetus-in-utero is usually protected from
maternal infections by the placental barrier.
However, human immunodeficiency virus
(HIV), rubella virus, cytomegalovirus,and
Toxoplasma gondii cross the placental barrier
and cause congenital infections.
 Cont.
• Very old people are susceptible to suffer more
than young people from a disease (e.g.,
pneumonia) and have high mortality.
• Measles, mumps, poliomyelitis, and chicken
pox are few examples of the diseases that
cause more severe clinical illness in adults
than in young children
 Nutritional Status
• Nutritional status of the host plays an important
role in innate immunity. Both humoral and cell
mediated immunities are lowered in malnutrition.
Examples are:
• Neutrophil activity is reduced, interferon response
is decreased, and C3 and factor B of the
complement are decreased in protein- calorie
malnutrition.
 Cont.
• Deficiency of vitamin A, vitamin C, and folic
acid makes an individual highly susceptible to
infection by many microbial pathogens.
 Hormonal Levels
• Individuals with certain hormonal disorders
become increasingly susceptible to infection.
• For example, individuals suffering from diabetes
mellitus, hypothyroidism, and adrenal dysfunction
are increasingly susceptible to staphylococcal
infection, streptococcal infection, candidiasis,
aspergillosis, zygomycosis and many other
microbial infections.
 Cont.
• Pregnant women are more susceptible to
many infections due to higher level of steroid
during pregnancy.
 Mechanisms of innate immunity
• Innate immunity of the host performs two
most important functions: it kills invading
microbes and it activates

• acquired (adaptive) immune processes

 Cont.
The innate immunity is primarily dependent on
four types of defensive barriers:

• Anatomic barriers

• Physiologic barriers

• Phagocytosis

• Inflammatory responses
 Anatomic barriers
• Anatomic barriers include skin and mucous
membrane. They are the most important
components of innate immunity.

• They act as mechanical barriers and prevent

entry of microorganisms into the body.
 Cont.
• The intact skin prevents entry of microorganisms.
For example, breaks in the skin due to scratches,
wounds, or abrasion cause infection.

• Bites of insects harboring pathogenic organisms

(e.g., mosquitoes, mites, ticks, fleas, and
sandflies), introduce the pathogens into the body
and transmit the infection.
 Cont.
• Skin secretes sebum, which prevents growth
of many microorganisms.

• The sebum consists of lactic acid and fatty

acids that maintain the pH of skin between 3
and 5, and this pH inhibits the growth of most
microorganisms
 Cont.
• Mucous membranes form a large part of outer
covering of gastrointestinal, respiratory,
genitourinary, and many other tracts of
human host.
 Nonspecific defense mechanisms
A number of nonspecific defense mechanisms act to
prevent entry of microorganisms through mucous
membrane, this include;
• Saliva, tears, and mucous secretions tend to wash away
potential invading microorganisms, thereby preventing
their attachment to the initial site of infections.
• These secretions also contain antibacterial or antiviral
substances that kill these pathogens.
 Cont.
• Mucus is a viscous fluid secreted by the epithelial
cells of mucous membranes that entraps invading
microorganisms.
• I•n lower respiratory tract, mucous membrane is
covered by cilia, the hair-like protrusions of the
epithelial cell membranes.
• The synchronous movement of cilia propels
mucus entrapped microorganisms from these
tracts.
 Cont.
• Nonpathogenic organisms tend to colonize the
epithelial cells of mucosal surfaces.

• These normal flora generally compete with

pathogens for attachment sites on the epithelial
cell surface and for necessary nutrients.
 Physiologic barriers
• The physiologic barriers that contribute to
innate immunity include the following:

• •
Gastric acidity is an innate physiologic barrier
to infection because very few ingested
microorganisms can survive the low pH of
stomach contents.
 Cont.
• Lysozyme, interferon, and complement are
some of the soluble mediators of innate
immunity. Lysozyme has antibacterial effect
due to its action on the bacterial cell wall.

• Interferons are secreted by cells in response to

products of viral infected cells.
 Cont.
• These substances have a general antiviral
effect by preventing the synthesis of viral
structural proteins.

• Complement is a group of serum-soluble

substances that when activated damage the
cell membrane.
 Phagocytosis
• Phagocytosis is another important defense
mechanism of the innate immunity.

• Phagocytosis is a process of ingestion of

extracellular particulate material by certain
specialized cells, such as blood monocytes,
neutrophils, and tissue macrophages.
 Inflammatory responses
• Inflammatory responses: Tissue damage
caused by a wound or by an invading
pathogenic microorganism induces a complex
sequence of events, collectively known as the
inflammatory responses.
 Cont.
• The four cardinal features of inflammatory
responses are redness, rise in temperature,
pain, and swelling.
• Mediators of inflammatory reactions:
Histamine, kinins, acute phase proteins, and
defensin are the important mediators of
inflammatory reactions.
 Histamine
• It is a chemical substance produced by a
variety of cells in response to tissue injury. It is
one of the principal mediators of the
inflammatory response.
• It binds to receptors on nearby capillaries and
venules, causing vasodilatation and increased
permeability
 Kinins
• These are other important mediators of
inflammatory response.
• They are normally present in blood plasma in
an inactive form.
• Tissue injury activates these small peptides,
which then cause vasodilatation and increased
permeability of capillaries.
 Cont.
• Bradykinin also stimulates pain receptors in
the skin.

• This effect probably serves a protective role

because pain normally causes an individual to
protect the injured area.
 Acute-phase proteins
• These include C-reactive proteins and mannose-
binding proteins that form part of the innate
immunity.

• These proteins are produced at an increased

concentration in plasma during acute-phase
reaction, as a nonspecific response to
microorganisms and other forms of tissue injury
 Cont.
• They are synthesized in the liver in response
to cytokines called proinflammatory cytokines,
namely, interleukin-1 (IL-1), interleukin-6 (IL-
6), and tissue necrosis factor (TNF).
• They are called proinflammatory cytokines
because they enhance the inflammatory
responses.
 Defensins
They are important component of the innate
immunity.
• They are cationic peptides that produce pores in
membrane of the bacteria and thereby kill them.
• The respiratory tract containsβ-defensins,
whereas the gastrointestinal tract contains α-
defensins.
• The α-defensins also exhibit antiviral activity.