This document discusses the different types of immunity and the body's defense mechanisms against infection. It describes two main types of immunity: nonspecific (innate) immunity and specific immunity. Nonspecific immunity provides the first line of defense and includes physical barriers like skin, chemical barriers like acids and enzymes, and cellular responses like phagocytosis. It explains several mechanisms of nonspecific immunity such as mechanical barriers, biochemical factors, cellular mechanisms, the role of normal flora, and inflammatory responses.

1. DR ABDULLAHI NOUR

2.  Learning Objectives At the end of this chapter,
students are expected to:
 - Describe the different types of immunity
 - Explain the role of the immunity in defense
mechanism
 - Discuss factor that affect the immunity.

3.  Definition Immunity can be defined as the
way in which the body can protect itself from
invasion by pathogenic microorganism and
provide a defense against their harmful
effect.
 Immunity is classified in to two major groups
- Non specific immunity
- Specific immunity

4.  Non-specific immunity, also called natural or innate
immunity, is the first line of defense against any
infectious agent.
 Non specific host responses provide an effective
barrier that prevents the microorganisms from
penetrating, inhibit or destroy the invader if it gains
access to the tissues, and eliminate or neutralize any
toxic substance elaborated by infectious agent.

5.  Several mechanisms are available in the
immunocompetent host.
 These include physical or mechanical barrier,
biochemical factors, cellular mechanism, role
of normal flora & inflammatory reactions.

6.  The unbroken skin and mucus membrane are
effective mechanical barriers to infectious
agents.
 The surface of the skin is also inhibitory to
the growth of most microorganisms because
of low moisture, low pH, and the presence of
secreted inhibitory substance.

7.  However, it is possible for some
microorganisms to enter the skin through
hair follicles, sebaceous glands or sweet
glands.
 Similarly, mucus membranes consist of an
epithelial layer and an underlying connective
tissue layer

8.  They line the entire digestive, respiratory,
urinary, and reproductive tracts.
 For example, the epithelial surface that lines
the nasal cavity and throat are protected by a
combination of mucous production and
cilliary movement.
 Because mucous is so
viscous,microorganisms adhere to it.

9.  Epithelial cells with cilia constantly move the
mucus layer to ward the mouth, where it
along with the trapped microorganism is
swallowed and eliminated.

10.  Besides, the action of coughing removes
mucus that contains microorganisms.
 In the urethra rapid flow of urine washes
away most microorganisms.
 Tear that wash the conjunctiva perform a
similar defensive function.

11.  These are chemical secretions produced by
the body that inhibit microbial growth. The
following are included as an example, keratin
is a skin protein produced by the outher most
cells of the skin, since it has very little water,
the skin becomes very dry and therefore to
most species of microorganism.

12.  The growth of microorganisms is inhibited in
the gastrointestinal tract by hydrochloric acid
and bile salt, which are secreted by the
stomach and liver, respectively.

13.  Lysozyme is an enzyme found in many body
fluids and secretions such as tears. It can
break down the cell wall of Gram-positive
bacteria and a few gram-negative bacteria by
hydrolyzing the peptidoglycan layer.

14.  Complement is a family of more than twenty
different proteins in serum that function as a
non-specific defense against infection.
 Interferons are small proteins produced by
eucarytic cells in response to viral infection.

15.  The virally infected cell produces interferon for a
few hours, even for a day, and it will excrete and
used by other cells.
 When these cells become infected with the same
or unrelated virus, the interferons cause the cells
to produce molecules that prevent replication of
the infecting virus.

16.  Alveolar macrophages like neutrophils and
natural killers remove particles and
organisms that enter the alveoli.
 Neutrophils are the first phagocytes in the
infected area that can non-specifically
phagocytize some microbes.

17.  Natural killer cells are large lymphocytes
whose function is to kill undesirable cells
such as tumor cells and virus infected cells.

18.  The human body is inhabited by a large
number of microorganisms, mainly bacteria,
which together, are called the body’s normal
flora or commensals.

19.  The term normal flora implies that such
microbial inhabitants are harmless For the
most part, normal flora microorganisms do
not cause disease.

20.  The commensally can stop the growth of
potentially pathogenic organisms through
different mechanism such as occupying
attachment sites and by producing substance
against pathogenic organism.
 They also compete for essential nutrients for
their growth.

21.  The inflammatory response is the vascular
and cellular reaction to the presence of
invading microorganisms or injury.
 It is one of the most effective defense
mechanism in human and other animals.

22.  The process of inflammation may be divided in
to the following stages:
 Initiation (Damage to tissue) ƒ
 Tissue response ƒ
 Leukocyte response ƒ
 Tissue repair (resolution) ƒCure.

23.  The damaged cells at the site of injury initiate
the tissue response by releasing chemical
factors such histamine, which in turn trigger
vasodilatation and increased permeability of
capillaries, permitting influx of fluids and
blood cells in to the site.

24.  Then, the phagocytic cells accomplish the
leukocyte response, by engulfing the
microbes and damaged tissue.

25.  In addition to destroying and removing an
injurious agent such as a microbe or its
products, the inflammatory response also
limits the effects of the agent or its products
by confining it or walling it of from the
surrounding tissues