1. Me and germs – a partnership?

Microbes live on many surfaces and in niches in the body. The most prolific population of bacteria, viruses, fungi and other microbes is in the gut, with trillions of organisms and thousands of different species, but large populations are also found on our skin, in our mouth and elsewhere. Mostly these microbes play a key role in our innate immune defence system, helping to prevent disease as well as, in the gut, digesting food and nutrients. They are our body’s key defence system against infections and disease. We need these microbes for survival, and they make us their home. This is a symbiosis: we each need each other to thrive.

Our symbiotic relationship with microbes evolves constantly, with more dangerous elements held in check within a wider ecosystem. However, harmful organisms (‘pathogens’) can break through the controls and the body’s innate defences. Pathogens include: prions (causing eg. mad cow disease, CJD and kuzu), viruses, bacteria, protozoa (causing eg. malaria and amoebic dysentery) fungi, and worms (eg. roundworm, hookworm, tapeworm, trichinosis, and schistosomiasis).

Barrier defences against pathogens are at every location and we refer to these often in this Immunity Issue. Invasion can occur through inhalation in the respiratory tract, ingestion of contaminated food in the digestive tract, via cuts or abrasions in the skin or through damaged mucous membranes (eyes, nose, mouth and throat).

One great example of barrier defence is in the airways. This has its own self cleaning mechanism called the mucociliary escalator. Most cells in the mucosal lining have whip-like hairs which all move in the direction of the throat and mouth. They support by a film of sticky mucus produced by other mucosal cells to create a form of moving flypaper! As fine particles, dust and microbes are inhaled they are sent by the structure of the airways into a vortex, the air is spun as it moves through the airways so that they are thrown by centrifugal force against the sticky surface and moved firmly to the exit! This escalator is of course totally quiet. If it is overwhelmed or compromised (by mucus being too profuse or sticky for example) then we fall back to more extreme measures, called the cough or sneeze.

We barely need to restate how important a barrier is our digestive system. In particular stomach acid and digestive enzymes sterilise our food unless they are compromised. Some people generally produce less acid than others and are prone to gut infections as a result. This can follow illness, chronic fatigue, anorexia, and drug and alcohol abuse, or simply too muuch antacid or prescription of omeprazole ir other treatments for excess acid.

If a pathogen breaches any of our barriers, our immune cells spring into action. Neutrophils are the body’s fast-acting first responders; macrophages engulf pathogens and alert other immune cells; T cells coordinate immune response and attack infected cells; B cells produce antibodies and natural killer cells destroy infected host cells. (see the previous posts on innate and adaptove immunity)

For a deep dive into these topics check out these key research papers:

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