BSI: Immunology Introduction

• ●Typical human pathogens include viruses,
• bacteria, fungi and multicellular parasites.

● This first line of defense is present at birth ("innate") and basically consists of external physical and chemical protective barriers of the skin and the exposed mucous membranes.

• ● In addition, the GI and respiratory tracts have
• specific defense mechanisms due to their constant exposure to our environment.

● The skin provides an excellent physical barrier which is very hard to penetrate unless broken or cut. The outer epidermis is principally composed of dead keratinized cells with plenty of tight junctions to limit access between them.

• ● Skin secretions principally from hair follicles
• (sebaceous glands) have a ↓pH and contain lactic and fatty acids which all inhibit bacterial growth (so non-hairy skin is more susceptible).

• ● Sweat, tears, saliva and some nasal secretions all
• contain the enzyme lysozyme which is capable of
• destroying the cell walls of certain bacteria. Salinity in sweat may also help.

• ● Modified sweat and sebaceous glands in the external auditory canal of the ear secrete cerumen (ear wax) which like mucus traps dust particles but
• also repels insects.

● The viscosity of mucus traps many organisms and prevents adherence prior to expelling (coughing, sneezing, etc.). In addition it contains antimicrobial peptides.

● Both urine (♀ and ♂) and vaginal secretions tend to wash potential pathogens away.

• ● After menstruation begins the vaginal secretions
• become slightly acidic which inhibits bacterial growth (but favors fungi/yeasts and therefore infections are common).

● Semen contains spermine and zinc which are lethal to some pathogens.

● Blood draining the GI tract filters first through the liver which is full of immune cells.

● However the main defense for the GI tract is stomach acid which has a pH ~1.5 due to HCl secretion plus the protease pepsin. The combination of these two chemicals kills virtually all pathogens entering the GI tract.

● Mother's milk contains the powerful enzyme lactoperoxidase which is antimicrobial (in addition milk contains antibodies from the mother to confer protection to the suckling child).

● Defecation and vomiting caused by some microbial toxins acting on smooth muscle of the GI tract or via the chemosensitive area of the brainstem helps expel the toxins and pathogens also.

● Mucus trapping microbes operates throughout the upper respiratory system from the nasal cavities down to the bronchioles. Cilia sweep the mucus out of these areas to be swallowed.

● Nose hairs are the first level of filtration. It is healthier to breath through our noses for this function plus warming and humidifying inhaled air.

● In addition immune cells such as macrophages enter the alveoli of the lungs searching for pathogens to phagocytize. In fact macrophages will travel across moist external bodily surfaces, such as the cornea of the eye, searching for pathogens.

● We live in harmony (“symbiosis") with many microbes (for example, gut flora/fauna that produce needed vitamin K). Other places this normally occurs is on the skin, in the mouth and throughout the GI tract.

● Disruption of these colonies may make it easier for potentially pathogenic bacteria to grow and colonize these areas for themselves.