DEFENSE MECHANISM
OF THE BODY
Prepared By
Ms. Beenish Tufail

What is defense mechanism
• The human body has three primary lines of defense to
fight against foreign invaders, including viruses, bacteria,
and fungi.
The immune system's three lines of defense include
• First line of defense
• Second line of defense
• Third line of defense

First line of defense:
physical and chemical barriers
• Prevent all types of foreign agents from penetrating the
outer layer of the body.
• No specific foreign agent is targeted at this level.

Physical Barriers
Skin
• Cells filled with keratin, making skin impenetrable,
waterproof, and resistant to disruptive toxins and most
invaders
• Dead cells are shed and replaced ( 1 million every 40
min) taking microbes with them
Mucous membrane
• The inner surface of body are guarded by mucous
membrane that line the respiratory, digestive, urinary, and
reproductive systems and protect the internal lining
Hair
• hair in the nose act as a coarse filter

Chemical Barriers
Sweat produced by glands in the skin, wash away
microbes and their acidity slows bacterial growth
Mucous membranes produce sticky mucous that traps
many microbes
Saliva and tears contain an enzyme called lysozyme that
kills bacteria by rupturing their cell walls
Cerumen (ear wax) produced in the ear canal and
protects the canal by trapping dirt and dust particles

Second line of defense:
Defensive cells
If pathogens penetrate the first line of defense, these cells
play a role in inhibiting or destroying the pathogen before
it harms the body
Phagocytes engulf pathogens, damaged tissues, or dead
cells
• Neutrophils
• Macrophages
Eosinophils discharge destructive enzymes to destroy
pathogen too big for phagocytes
Natural killer cells seek out abnormal cells

Defensive proteins
Interferon protein infected cells produced interferon
• Binds with other cells that become infected with a virus,
and protects it by stimulating the cells to produce antiviral
proteins that prevent the virus from making copies of itself
• Interferon attracts and stimulate natural killer cells and
macrophages to kill cells infected with the virus

Inflammation
• When body tissues are injured or damaged, a series of
events called the inflammatory response occurs
Redness caused by increased blood flow to the damaged
area
Heat increased blood flow elevates the temperature in the
area of injury, increasing metabolic rate of the body cells
Swelling histamine makes capillaries more permeable than
usual
Pain causes person to protect the area and prevent
additional injury

Fever
• A fever is an abnormally high body temperature caused
by pyrogens (chemicals that set the thermostat in the
brain to a higher set point)
• A mild or moderate fever helps the body fight bacterial
infections by slowing the growth of bacteria and
stimulating body defense response

Third line of defense:
The Immune System
• Immunity means protection from infections
• When the first two line of defense of the body can not
prevent the infection, the immune system acts to eliminate
the infectious agent and prevent the body from infection
• Immune response consists of two steps
• Step 1 recognition of the pathogen or foreign molecule
• Step 2 mounting reaction against pathogen

Natural immunity
• It is resistance to a disease possessed by an individual.
• Nature has given certain individuals, species and races
immunity against certain diseases.
• For example, some individuals are more resistant to
certain infections than others

Acquired immunity
• Only with natural immunity it is not possible to fight off
each and every infection and survive.
• Immunity is induced by certain ways. Acquired immunity is
developed during persons lifetime
• It is not inherited
• This immunity can be acquired actively or passively

Active immunity
• Naturally acquired active immunity is acquired when a
person is exposed to natural infections or to some antigen
in the day to day life. Following the exposure the immune
system responds by producing antibodies
• Artificially acquired active immunity is acquired by
administering specially prepared antigens which produced
specific antibodies. This is also known as vaccination

Passive immunity
• Naturally acquired passive immunity is involves natural
transfer of antibodies from a mother to fetus via placenta
or breast milk and thus providing immunity to the newborn
for a few days to few months.
• Artificially acquired passive immunity antibodies are
directly administered to body for stimulation of immune
responses. These antibodies are either produced in
animals or in human.

Cont…
• Immunity can be classified into two categories
• Innate immunity
• Adaptive immunity
Innate immunity reflects the first and second line of
defense against infections
it is the defense mechanism which are present as inherent
Adaptive immunity consists of defense mechanism which
are stimulated by exposure to infectious agents

Adaptive immunity
• Humoral immunity is mediated by antibodies produced
by B lymphocytes ( matured in bone marrow)
It is principal defense mechanism against extracellular
microbes and their toxins
• Cell mediated immunity is mediated by cytokines,
produced by T lymphocytes ( matured in Thymus Gland)
defense mechanism against intercellular microbes

Resistance and susceptibility
• Susceptibility means they can't grow if the drug is
present. This means the antibiotic is effective against the
bacteria.
• Resistance means the bacteria can grow even if the
drug is present.

Phagocytosis
• The process by which a phagocyte (a type of white blood
cell) surrounds and destroys foreign substances (such as
bacteria) and removes dead cells.
• Bacteria, dead tissue cells, and small mineral particles are
all examples of objects that may be phagocytized.

Process of phagocytosis