2. Immunity
Immunity: ability of body to protect itself from foreign
substances and cells, including disease-causing agents.
The immune system: is Defense body mechanism : All
the cells (and proteins) in the body that protect the body
against foreign organisms and substances and also against
cancerous cells. The immune system must be able to
differentiate between material that is a normal component
of the body ("self") and material that is not native to the
body "nonself.
3. Introduction
Types of Immunity: The main function of the immune
system is to prevent or limit infections by pathogenic
microorganisms, such as bacteria, viruses, parasites, and
fungi. The recognition of microorganisms and foreign
substances is the first event in immune responses of a host.
The body’s defense mechanisms can be divided into:
(a) innate (natural) immunity and (b) acquired (adaptive)
immunity.
4. Types of Immunity
Innate Immunity: Innate immunity is the resistance that an
individual possesses by birth. Innate immunity may be
classified as (a) individual immunity, (b) racial immunity,
and (c) species immunity.
5. Types of innate 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.
6. Types of innate Immunity
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. 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 glucose6-
phosphatase dehydrogenase are also less susceptible to
infection by P. falciparum.
7. Types of innate Immunity
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.
8. Factors influencing innate immunity
The factors that may influence innate immunity of the host
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. 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.
9. Factors influencing innate immunity
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.
This may be due to more active immune response in an
adult causing greater tissue damage
10. Factors influencing innate immunity
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. ■ Deficiency of
vitamin A, vitamin C, and folic acid makes an individual
highly susceptible to infection by many microbial
pathogens.
11. Factors influencing innate immunity
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. Similarly, pregnant women are
more susceptible to many infections due to higher level of
steroid during pregnancy.
12. 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.
Innate immunity unlike adaptive immunity, however, does
not have any memory and does not improve after re-
exposure to the same microorganism. The innate immunity
is primarily dependent on four types of defensive barriers:
(a) anatomic barriers (b) physiologic barriers
(c) phagocytosis and (d) inflammatory responses.
13. Mechanisms of innate immunity
A -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.
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. Skin secretes sebum, which prevents
growth of many microorganisms.
14. Mechanisms of innate immunity
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. Mucous
membranes form a large part of outer covering of
gastrointestinal, respiratory, genitourinary, and many other
tracts of human host. A number of nonspecific defense
mechanisms act to prevent entry of microorganisms through
mucous membrane.
15. Mechanisms of innate immunity
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.
16. Mechanisms of innate immunity
Mucus is a viscous fluid secreted by the epithelial cells of
mucous membranes that entraps invading microorganisms.
In 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.
17. Mechanisms of innate immunity
In addition, 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.
18. Mechanisms of innate immunity
B- 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.
Lysozyme, interferon, and complement are some of the
soluble mediators of innate immunity.
19. Mechanisms 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. 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.
20. Mechanisms of innate immunity
C- 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. It is a type of
endocytosis in which invading microorganisms present in
the environment are ingested by the phagocytic cells. In this
process, plasma membrane of the cell expands around the
particulate material, which may include whole pathogenic
microorganisms to form large vesicles called phagosomes.
21. Mechanisms of innate immunity
D- 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. The end result of inflammation
may be the activation of a specific immune response to the
invasion or clearance of the invader by components of the
innate immune system. The four cardinal features of
inflammatory responses are (redness), (rise in
temperature), (pain), and (swelling).
22. Mechanisms of innate immunity
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.
23. Mechanisms of innate immunity
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. Brady kinin 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.
24. Mechanisms of innate immunity
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. They are synthesized in the liver in response to
cytokines called proinflammatory cytokines, namely,
interleukin-1 (IL-1), interleukin-6 (IL6), and tissue necrosis
factor (TNF). They are called proinflammatory cytokines
because they enhance the inflammatory responses.
25. Mechanisms of innate immunity
Defensins: They are another important component of the
innate immunity. They are cationic peptides that produce
pores in membrane of the bacteria and thereby kill them.
These are present mainly in the lower respiratory tract and
gastrointestinal tract. The respiratory tract contains beta -
defensins, whereas the gastrointestinal tract contains
Alpha-defensins.
26. Mechanisms of innate immunity
the alpha-defensins also exhibit antiviral activity. They bind
to the CXCR4 receptors and block entry of HIV virus into
the cell. How these defensins differentiate microbes from
some cells is not known.
