Nonspecific Defenses of the Host

3. Describe the first line of defense.
4. List the mechanical factors involved.
5. List the chemical factors involved.

Skin and Mucous Membranes

Mechanical Factors

The structure of intact skin and the waterproof protein keratin provide resistance to microbial invasion.

Some pathogens, if present in large numbers can penetrate mucous membranes.

The lacrimal apparatus protects the eyes from irritating substances and microorganisms.

Saliva washes microorganisms from teeth and gums.

Mucus traps many microorganisms that enter the respiratory and gastrointestinal tracts; in the lower respiratory tract, the ciliary escalator moves mucus up and out.

The flow of urine moves microorganisms out of the urinary tract, and vaginal secretions move microorganisms out of the vagina.

Chemical Factors

Sebum contains unsaturated fatty acids, which inhibit the growth of pathogenic bacteria. Some bacteria commonly found on the skin can metabolize sebum and cause the inflammatory response associated with acne.

Perspiration washes microorganisms off the skin.

Lysozyme is found in tears, saliva, nasal secretions, and perspiration.

The high acidity (pH 1.2-3.0) of gastric juice prevents microbial growth in the stomach.

Transferrins bind iron.

Normal microbiota prevent the growth of many pathogens.

Normal Microbiota and Nonspecific resistance

Normal microbiota change the environment, which can prevent the growth of pathogens.