INTRODUCTION
TO
IMMUNOLOGY:
DEFINITIONS
Immunity:
resistance to disease, specifically infectious
disease and tumors.
The immune system:
the collection of cells, tissues, and molecules
that mediate resistance to infections.
The immune response: the coordinated
reaction of these cells and molecules to
infectious microbes.
History
Documents show
that as early as AD
1000,
the Chinese custom
existed of making
children inhale
powders made from
the crusty skin
lesions of patients
recovering
from smallpox.
Edward Jenner noticed that milkmaids were
generally immune to smallpox.
Jenner proposed that cowpox can be used to
immunize children against small pox virus
(small pox vaccine).
On 14 May 1796, Jenner tested his hypothesis
by inoculating an eight-year-old boy by pus
from cowpox blisters on the hands of a
milkmaid who had caught cowpox from a cow.
The boy was later challenged with small pox
material and showed no sign of infection.
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In 1876; Robert Koch transmitted anthrax from
in vitro culture to animals; he proved that the
microbe is the causative agent of disease.
Louis Pasteur (1881), developed the first three
veterinary attenuated vaccines: chicken
cholera, anthrax and rabies.
In 1883, Metchnikoff observed the
phagocytosis of fungal spores by leukocytes
and advanced the idea that immunity was due to
WBCs.
Tissues And Organs Of The Immune System
 Primary lymphoid organs (development &
education)
Bone marrow.
Thymus.
• Secondary lymphoid organs (sites where
immune cells make contact with each another)
Spleen.
Lymph nodes.
Mucosal associated lymphoid tissues (MALT)
e.g. tonsils.
THYMUS & BONE MARROW
BARRIERS TO INFECTION:
Human body has several barriers:
• Physical barriers
• Chemical barriers
• Biologic barriers
These barriers provide the first line of defense
against the entry of microbes.
The Physical Barriers:
o
o
o
o
Skin and Mucus membranes
Respiratory tract
Urinary tract
GIT.
SKIN:
The initial mechanical barrier.
• The outermost layer of epidermis
(stratum corneum), is composed of
dead tightly layered squamous cells.
This layer is an inhospitable dry
surface.
• Continuously dividing keratinocytes
provides a constant detachment of
squamous cells and microbes.
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Mucous Membranes:
•
The epithelium of mucous membranes
line the body cavities.
•
This epithelium contains goblet cells that
secrete mucus.
•
Mucus viscosity traps the inhaled
microbes.
•
In GIT, the mucus protects the epithelial
cells and underlying tissue from damage
by digestive enzyme.
Urinary Tract:
The flashing action of urine (urination) washes
away pathogens and prevent urinary tract infections.
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Respiratory Tract:
o The mucus trap invading microbes.
o The hair-like rhythmically beating cilia of the
epithelia lining the respiratory tract passages
remove the secretions.
o Alveolar macrophages play an important role
in defence against microbes.
GIT: intestinal peristalsis.
THE CHEMICAL BARRIERS:
o Skin
o Gastrointestinal Tract (GIT)
o Respiratory Tract and Lacrimal
Secretions.
Skin:
• Sweat has slightly acidic PH of 5.5 and
contains lysozyme that breaks down the
bacterial peptidoglycan.
• The RNAses and DNAses of skin destroy the
microbial genetic material.
• Several antimicrobial peptides e.g. α & β
defensins which are induced by skin damage,
inhibit microbial growth causing lysis.
• The fatty acids and sebum (lipids) have
antimicrobial action.
The Gastrointestinal Tract:
• The highly acidic environment of stomach (pH of 1
to 3) protects the intestines.
• The secreted antimicrobial molecules (α defensine
and cryptidin) of GIT destroy some pathogens.
• The digestive action of the enzymes.
Respiratory tract and lacrimal secretions:
• In respiratory tract: β defensin and IgA has
antimicrobial activity.
• Production of tears; which contain IgA and
lysozymes (protect the eye from pathogens).
The Biologic Barriers:
Commensal Microbes (normal flora):
Microorganisms that exist in a symbiotic
relationship with the body.
How do commensals inhibit pathogenic
colonization?
o Production of bacteriocins (antibiotics).
o Competitive depletion of essential
nutrients.
o Production of toxic products (vaginal lactic
acid PH 4).
o Stimulation of natural antibodies.
Examples of common commensal Bacteria
Body Area
Common Commensal
o Skin
o Staphylococcus species
o Upper respiratory
tract, mouth, and
throat.
o Streptococcus (alpha
hem.) Neisseria
species.
o Intestinal tract
o Bacteroides and
Escherichia coli.
o Genital tract
o Lactobacillus species
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Establishment of Infection:
Infectious diseases occur when a pathogenic
organism invades human body barrier.
This can be related to the following factors:
o The microbial virulence
o The pathogenic dose.
o The port of entry.
o The host immunity.
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Pathogenic Dose:
It is the minimum number of organisms
required to establish an infection.
Virulence Factors :
Are microbial extracellular structures, proteins,
enzymes and toxins that enable the
microorganism to be a pathogen.
Ports of Entry:
o Ingestion.
o Inhalation
o Direct penetration
Examples:
Ingestion:
•Bacteria: Salmonella.
•Parasites: Entamoeba histolytica.
•Viruses: Hepatitis A.
Inhalation:
•Bacteria: Mycobacterium tuberculosis
•Fungi: Histoplasma.
•Viruses: influenza
Direct penetration:
•Trauma: Clostridum tetani.
•Needle stick: hepatitis B.
•Arthropod bite: Malaria
•Sexual transmission: Neisseria gonorrhoeae.
•Transplacental: Rubella virus.
• Skin penetration: Schistosoma.