Our immune systems have some non-specific first line of defense which are employed to ward off infectious agents before they penetrate our skins and mucous membranes. They are said to be non-specific because they carry out their jobs irrespective of the type of invader, be it bacteria, viruses, fungi or parasites.
In this episode, we walk through the barriers involved and how they shield us from infections through some mechanical and chemical processes.
These barriers include the skin and mucous membranes that resist the entrance of the microorganisms by removing them. We are born with them and they are normally part of the body’s anatomy and physiology.
📌THE SKIN: the skin is a common portal of entry for microbes because of its exposure.
📜Mechanical aspect: the skin is the largest organ in terms of surface area and weight and is composed of skin cells that are jumbled and cemented together with the protein keratin. This protein holds the skin cells together and keep them from pulling apart, forming a barrier that protects us from foreign invaders. In its unbroken state, it is usually not penetrated by pathogens and its periodic shedding of the top layer helps remove microbes at the surface. Normal activities such as perspiration washes microbes away from the skin surface.
📜Chemical aspect: some microorganisms can enter the skin and to do that, they have to go through the sebaceous gland. For some, this entrance is inhibited by fatty acids present in sebum which makes the skin acidic and unfriendly. Lysozyme, an antimicrobial enzyme on the skin devour microbes. An abundant antimicrobial known as psorasin is also found on healthy skin.
📌THE MUCOUS MEMBRANES: the mucous membranes are important components of the first line of defense, lining tracts such as the respiratory tract, genitourinary tract and gastrointestinal tract. They secrete a fluid called mucus which prevent drying out. These membranes are permeable but their fluids such as mucus, saliva and tears (in mucous membrane of the eyes) get rid of microbes.
📗Respiratory tract: this is a portal for the entrance of microorganisms because the air we breathe can contain them.
📜Mechanical aspect: it has a thin film of mucus covering it, starting from the nose. Microbes are trapped in mucus and the hair-like cilia in this tract move them upwards, preventing their entrance into the lungs. This upward movement lead to cough and sneezing which shows that foreign particles have entered the nose (which is also connected to our mouths) and are trapped in mucus. We then cough and sneeze to eject these irritants.
📜Chemical aspect: the mucous membrane contains lysozyme which breakdown microorganisms.
📗Gastrointestinal tract: because we eat and drink, this serves as a portal for microbes which are in foods.
📜Mechanical aspect: peristalsis, defecation, vomiting and diarrhoea help to expel microbes in the gastrointestinal tract. Peristalsis push food along with microbes down the tract. Mass propelling of the contents in the large intestine result in defecation ( expelling microbes). In response to microbial toxins, the muscles of the tract contract vigorously, resulting in vomiting and/or diarrhoea, which may also expel microbes.
📜Chemical aspect: the gastrointestinal tract has factors for elimination of pathogens, starting from the mouth which contains saliva. The saliva is made up of hydrolytic enzymes such as lysozyme which is an enemy to microbes. The stomach which is acidic contains gastric juice which is a mixture of hydrochloric acid, enzymes and mucus which are also enemies to the nature of some microbes such as Vibrio cholerae which normally thrives in alkaline milieu. The small intestine contains proteolytic enzymes.
The mucous membrane of the GIT also have numerous normal flora which can compete and inhibit pathogens. This is called interference or bacterial interference. Bacterial interference can be established in people with insufficient normal flora by supplying them normal flora from healthy individuals.
📍NB: the normal flora is not considered part of the innate immune system, but they are discussed because of the considerable protection they afford. This protection can come in three ways: competition for space and nutrients with pathogens, secretion of bacteriocins and development of the immune system after birth.
📗Genitourinary tract: the urethra and vagina are potential portals for entrance of microorganisms.
📜Physical aspect: periodic flushing of the urethra by the flow of urine prevents microbial colonization in the genitourinary tract. Vaginal secretions likewise flush away microbes from the female body.
📜Chemical aspect: Urine contains lysozyme and has an acidic pH that inhibits microbes. The vagina is also acidic due to the presence of lactic acid which inhibits microbes
ADD-ONS: ear wax also serve as a mechanical barrier which traps microbes and also as a chemical barrier because it contains sebum which constitute fatty acids giving the ear canal a low pH.
The barriers discussed so far help prevent microbes from entering the skin and mucous membrane but just in case they persist and evade these barriers, the body’s immune system has a lot more in store for them with another line of defense which will be discussed in an upcoming series.
Reference material: Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. 2019. Microbiology: an introduction
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