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

Edward Jenner studied the immunity of the farm workers when they carried grease disease; (cowpox) from horses hooves to cow’s nipples.

Jenner proposed that cowpox can be used to immunize children against small pox virus (small pox vaccine)

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 recoveringfrom smallpox

Anthrax was first transmitted from in vitro culture to animals by Robert Koch in 1876 ; he proved that the microbe is the causative agent of disease.

Louis Pasteur (1881), developed the first three 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

Bone marrow. Thymus.

• Secondary lymphoid organs Spleen. Lymph nodes.

• Mucosal - Associated Lymphoid Tissues (MALT)

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 Skin

o Mucus membranes

o Respiratory tract

o Urinary tract

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:

• 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 sterile urine

(urination) washes away pathogens and

prevent urinary tract infections.

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Respiratory Tract:

oThe hair-like rhythmically beating cilia of the epithelia lining the respiratory tract passages remove the secretions containing trapped microbes.

o The mucus production and alveolar macrophage play an important role in defence against microbes.

THE CHEMICAL BARRIERS:

o Skin

o Gastrointestinal Tract (GIT)

o Respiratory Tract and Lacrimal

Secretions.

skin: • The sweat and sebaceous glands

play an important role of natural defense.

• 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.

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• Several antimicrobial peptides e.g. defensins which are induced by skin damage, inhibit microbial growth causing lysis.

• The unsaturated fatty acids and sebum 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.

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Respiratory tract and lacrimal secretions:

• In respiratory tract: β defensin 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.

Commensals are considered as important part of natural defense because, they protect human body cavities from colonization with exogenous pathogenic organisms.

Examples of common commensal Bacteria

Body Area

o Skin

o Upper respiratory tract, mouth, and throat.

o Intestinal tract

o Genital tract

Common Commensal

o Staphylococcus species

o Streptococcus (alpha hem.) Neisseria species.

o Bacteroides and Escherichia coli.

o Lactobacillus species

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.

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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 pathogenic dose.

o The microbial virulence .

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:

ingestion.

Inhalation

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.

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