Immune System
&
Immunity
By
Dr. Khaled Ibrahim
AT THE END OF THIS SESSION, STUDENT
SHOULD BE ABLE TO:
a. Define immunity and contrast the difference between its 2 types
(innate & acquired).
b. Describe some mechanisms for innate immunity (interferon’s &
complement should be included).
c. Describe briefly the mechanism of acquired immunity and
identify the major differences between its two types (humoral &
cell-mediated).
d. Relate the knowledge acquired to some clinical problems (e.g.,
AIDS)
Definition:
Immunity is Physiological defenses by which the body (the “host”)
destroys or neutralizes foreign matter, both living (microbes) and
nonliving (foreign bodies).
Thus, immune system has 3 functions:
(1) protects against infection by microbes — viruses, bacteria,
fungi, and parasites;
(2) Isolates or removes non-microbial foreign substances; and
(3) Destroys cancer cells that arise in the body, a function known as
immune surveillance.
 The cells that carry out immune defenses collectively make up the
immune system.
- They are a diverse collection of cells found in blood and in tissues
throughout the body.
- The major cells forming the immune system are called “white
blood cells” (WBCs) OR leucocytes.
WHITE BLOOD CELLS (LEUKOCYTES)
Leukocytes are the mobile units of the body's protective system
 Total leukocyte count: 4.000 – 11.000 /mm3
 Types:
depending
on
presence of
granules in
cytoplasm,
leukocytes
are divided
into:
Immune
organs
Immune defenses can be classified into 2 types which usually
interacts:
Nonspecific immune
defenses
(Innate Immunity)
Specific immune
defenses
(Acquired Immunity)
 Protect against microbes
 Protect against microbes to
or F.B. (invaders) without
which the body is previously
having to recognize their
exposed (recognized before)
specific identity.
either through infection or
 The mechanisms used are
immunization.
not specific to any invader.
 The mechanisms used are
specific for each invader.
Innate Immunity
Acquired Immunity
Specificity
Non-specific
Specific
i.e., Reacts to invaders from i.e., Reacts to a specific
invader each time and can
different type
not react to another.
Diversity
Limited
Very high
i.e., Although the cells are of
the same type, each group
shows
totally
different
surface molecules.
Memory
No
Yes
Remembers the invader on
re-exposure.
Non-reaction to self
Yes
Yes
Components
Cells
Phagocytes & natural killer Lymphocytes
cells
Molecules
Complement
Antibodies
Mechanisms of Innate Immunity
I - Defenses at Body Surfaces
Very few microorganisms can penetrate the intact skin,
Various skin glands & tears contain anti-microbial agents
Mucus secreted by epithelial lining contain antimicrobial agents and sticky
to which microbes adhere.
Hairs at the entrance of nose
Cough and sneezing reflex
Acid secretion by the stomach and uterus
II- Inflammation
1. Initial entry of bacteria into tissue
2. Vasodilation in the infected area, leading to increased blood flow
3. Marked increase in protein permeability of the capillaries and venules in
the infected area, with resulting diffusion of protein and filtration of fluid
into the interstitial fluid.
4. Chemotaxis: exit of leukocytes from the venules into the interstitial fluid
of the infected area
5. Destruction of bacteria in the tissue either through phagocytosis or by
mechanisms not requiring phagocytosis.
6. Tissue repair
III- Complement
family of plasma proteins which is involved in:
1) killing of microbes without prior phagocytosis.
2) Opsonization: making phagocytosis easier.
3) Chemotaxis: Direction of phagocytes toward the source of infection.
opsonization
IV- Interferons
are a family of cytokines that nonspecifically inhibit viral replication inside host
cells
Antigen (immune stimulant)
Is any foreign substance which when introduced into the body ,is
capable to stimulates specific immune response.
Antigen nature:
either proteins or high molecular weight polysaccharides (8000
or greater) that are specific for each type organism.
Basic types of acquired immunity
1) Humoral immunity
2) Cell-mediated immunity
= B cell immunity
= T cell immunity
1. Performed by B-lymphocytes 1. Performed by T-lymphocytes
2. It's a major defense against 2. It's a major defense against
bacterial infection
viral & fungal infection
3. The B-lymphocytes produce 3. The T-cell them selves can
kill the abnormal cells.
antibodies that kill bacteria
4. It is also responsible for
allergic
reactions
and
rejection of transplanted
organs.
Acquired Immunity involves the following steps:
1) Antigen recognition and presentation by “antigen presenting
cells”.
2) Lymphocyte activation and differentiation.
3) Elimination of antigen by:
a) Antibodies secreted by plasma cells (differentiated from Blymphocytes).
b) Direct attack by activated T-cytotoxic cells.
Functions of different types of T-lymphocytes
1) Helper T-lymphocytes (TH cells):
- They are the most numerous of the T-cells (75%).
- They serve as the major regulator of all immune functions. They do this by
forming a series of protein mediators, called lymphokines, that act on other cells
of the immune system as well as on the bone marrow cells.
2) Cytotoxic-T cells (Tc) or killer cells
-Tc is a direct attack cell that is capable of killing micro-organisms and some of
the body’s own cells specially when they are cancerous or invaded with viruses.
They are also responsible for rejection of transplants of foreign tissues.
3) Suppressor T-lymphocytes (Ts cells)
-Ts cells are capable of suppressing the functions of both cytotoxic and helper T
cells.
N.B.:
TH-cell is the type that is attacked and destroyed by the acquired
immunodeficiency syndrome virus (AIDS virus), which leaves the
body almost totally unprotected against infectious diseases.