Every day we are exposed to a range of physical, chemical and infectious challenges. This is nothing new for living things, and during evolution many defence strategies have emerged to enable living organisms to avoid or cope with these potential dangers to health. Since as far as we know all living things on our planet share a common heritage and chemistry, this creates a dilemma that the body must solve: how can it distinguish between what is meant to be there in the body - the cells and chemicals such as hormones and nutrients and waste products - and what has come in uninvited from outside, and which could potentially be damaging? This distinction is sometimes expressed as 'self' and 'non-self' - self belongs and non-self must be recognised and disposed of before it causes harm. The conditions inside the body are so warm, comfortable, and inviting that it is not surprising that other smaller organisms would like to live there.
Defending our borders
The country we live in has a well-defined border and rules apply to those who might wish to enter from outside. There is a process of selection. To ensure the rules are adhered to, there are defence forces, border patrols, and policing, all overseen by government agencies. As a recognised member of the society we can expect certain privileges, but we will also have responsibilities, including living within the system of law.
Our biological body also has a boundary, and again there are rules applied to determine what can enter the body and what cannot. The body has numerous systems in place to ensure that the tissues are protected against the invasion of ‘non-self’ from outside (microorganisms, viruses, parasites, harmful chemicals etc).
Let us consider some of the strategies used by the body to keep itself free from potentially invasive and damaging organisms.
Firstly, the boundary between the outside and inside of the body. The tissues which make up the body are enclosed within membranes and epithelia which form physical barriers to entry. Clearly the skin is one of those barriers, but we must not forget other surfaces more deeply placed where there is also an interface between inside and outside, for example the digestive tract and the airways.
Remember that the external surface of the skin is colonized by microorganisms. Most of these are friendly in the sense that they do not cause disease, and indeed they are beneficial since their presence ensures that more harmful microorganisms cannot gain a foothold. The outer layers of the skin are comprised of dead cells, and they are being shed continuously as small flakes. This loss of skin flakes helps to keep the commensal microorganisms in balance - they are being cast adrift like penguins on an ice floe that has broken away from the edge of the sea ice. To compensate for this continuing loss of surface layers, new skin cells are constantly being formed in the deepest layer of the epidermis. Sweat glands and sebaceous glands release their secretions onto the skin and into hair follicles and these secretions have antibiotic properties, further helping to regulate the commensal microorganisms.
The skin has many additional functions that are linked with protecting the body, for example sensory nerve endings in the skin relay information to the brain about pain, touch, temperature, pressure and so on, so that the brain can organise effective responses to different situations. The skin protects against undue fluid loss. It also protects against harmful chemicals that come into contact with the body surface, and limits the damaging effects of ultraviolet radiation from the sun by developing pigmentation. The skin resists abrasion and heals when damaged.
The eyes are protected by the eye lids, eye lashes, eye brows, and antibacterial tears that wash and clean the front of the cornea and sclera.
The airways and the delicate airsacs in the lungs where gas exchanges take place are potentially vulnerable to invasive microorganisms. There are several defence mechanisms at work at this interface between outside and inside. Nasal hairs help to prevent larger objects such as insects entering the airways. Friendly microorganisms colonise the nasal cavities and displace potentially harmful bacteria. The trachea and bronchi are lined by a ciliated and mucus-secreting epithelium, and the mucus traps particulate matter and microorganisms before they reach the airsacs. The film of sticky mucus is gradually wafted up the airways towards the pharynx where it can be swallowed to be neutralised in the stomach acid, and fresh mucus is continuously formed to replace it.
The lining of the digestive system is also an interface between inside and outside. Even when we swallow food or liquid, it is not yet within the body - it must first be digested and absorbed before it gains entry. The food and fluids we take in may be accompanied by potentially harmful microorganisms, so the digestive tract has several ways of dealing with them. Firstly, our saliva contains antibodies which can neutralize some bacteria. Then, the concentrated hydrochloric acid in the stomach kills other microorganisms. The cells which line the stomach and the intestines which follow are protected by a layer of mucus, and within the lumen of the intestines there is a diverse microbiome of many different types of commensal microorganisms which generally prevent harmful bacteria from proliferating and also have other functions related to digestion and nutrition and schooling of the immune system.
There is the possibility of microorganisms gaining access to the urinary system from the outside, travelling along the urethra towards the bladder. This risk is reduced by the chemical make-up of urine and by the flushing of the pathway with sterile urine during urination.
A similar situation exists for the female reproductive tract. Potentially there is a pathway from the outside to the peritoneal cavity in the abdomen via the vagina, uterus, and fallopian tubes. However, the entrance to this pathway is protected by the low pH in the vagina which inhibits microorganism growth and by the mucus closing the cervix of the uterus.
Despite all of these measures, sometimes harmful agents can gain access to our tissues from the outside, perhaps when the boundary is breached at some point by injury or an operation. When this happens, body defences within the tissues come into operation. We shall consider these next.