Introduction to Immunology: Immunology began as a branch of microbiology; it grew out of the study of infectious diseases and the body’s response to them. Edward Jenner studied the immunity of the host (farm workers) when they carried (grease disease; Cowpox virus) from horse hooves to the cow’s nipples. Jenner proposed that Cowpox could be used for immunization of children against Small pox virus.

Edward Jenner; Discovery of small pox vaccine(1798). n

n Anthrax was first transmitted from in vitro culture to animals by Robert Koch in 1876 ; he proves 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.

Barriers to infection: Human body has several physical, chemical, and biologic barriers that provide the first line of defense against the entry of microbes. The Physical Barriers : 1- Skin: -The initial mechanical barrier. -The outermost layer of epidermis (Stratum corneum), is composed of dead tightly layered squamous cells. -This layer provides a microbe-inhospitable dry surface.

n - Continuously dividing keratinocytes provides a constant detachment of squamous cells and microbes.

n 2-Mucous membranes: -The epithelium of mucous membranes lines all of the body’s cavities. -This epithelium contains goblet cells that secrete mucus. -The mucus viscosity traps the inhaled microbes. -In GIT, the mucus protects the epithelial cells and underlying tissue from damage by digestive enzyme.

n 3-Respiratory tract: -The hair-like rhythmically beating cilia of the epithelia lining the respiratory tract passages remove the secretions containing trapped microbes. - The mucus production and alveolar macrophage. 4-Urinary tract: -The flashing action of sterile urine (urination).

The chemical barriers: 1- Skin: -The skin Sweat glands (sebaceous ) 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.

N -Several antimicrobial peptides (alpha and beta defensins) inhibit microbial growth causing lysis. -Finally, the unsaturated fatty acids have microcidal action. 2-The Stomach and gastrointestinal tract: -The highly acidic environment of stomach (pH of 1 to 3) protects the intestines. -The secreted microcidal molecules (alpha defensine and cryptidin) of GIT destroy the pathogens. -The digestive enzymes action.

n 3- Respiratory tract and Lacrimal secretions: -In Respiratory tract: Beta-Defensin has microcidal activity. -Production of tears; which contain IgA and lysozymes (protect the eye from pathogens).

Barriers to infection: n

The Biologic Barriers: (Commensal microbes): Commensal microbes are non-pathogenic microbes that exist in a symbiotic relationship with the body. Examples: Body area Common commensal microbe 1-Skin Staphylococcus species. 2-Upper respiratory Streptococcus (alpha hem.) tract, mouth, Neisseria species. and throat. 3-Intestinal tract Escherichia coli, and Bacteroides. 4-Urogenital tract Lactobacillus species.

Commensalism and microbial Competition: Commensal microbes are considered as important part of natural defense because, they protect human body cavities from colonization with exogenous pathogenic organisms. How Commensal microbes inhibit pathogenic colonization? 1-Production of bacteriocins. 2-Competitive depletion of essential nutrients. 3-Production of toxic by-products ( vaginal lactic acid pH 4). 4-Stimulation of natural antibodies.

Establishment of infection: An infectious disease occurs when a pathogenic organism Invades human body barrier. This may be directly related to the following factors: 1-The pathogenic dose. 2-The microbial virulence. 3-The Port of Entry. 4-The host immunity.

n Pathogenic dose: It is the minimum number of organisms that required to establish an infection. Virulence factors : are microbial extracellular structures, proteins, enzymes and toxins that increase the pathogenic ability of microbes. Port of Entry: 1-Ingestion. 2-Inhalation. 3-Direct penetration.

n Examples: Ingestion: Bacteria: Salmonella. Parasites: Entamoeba histolytica. Viruses: Hepatitis A. Inhalation: Bacteria: Mycobacterium Fungi: Histoplasma. Viruses: influenza virus.

n Direct penetration: Trauma: Clostridum tetani. Needle stick: hepatitis B. Arthropod bite: Malaria Sexual transmission: Neisseria gonorrhoeae. Transplacental: Rubella virus. Skin penetration: Schistosoma.