When it comes to infections, the body has its own mechanisms to fend them off. Some of these mechanisms such as the immune mediated resistance could be considered specific to a particular invasive pathogen. However, there are mechanisms that would act against most infective pathogens and other irritants without differentiating between them. These mechanisms are therefore given the name ‘non-specific’ defense responses and mechanical barriers to infections.
The skin as a mechanical barrier
The discussion in relation to these barriers and non-specific defense responses should start with the largest organ in the body, which is the skin. The skin consists of multiple layers of cells out of which the top most layers contain dead cells and a protein known as ‘keratin’. Together, these cells and keratin can prevent many infective and non-infective organisms from penetrating into the human body and most importantly into the blood circulation. Therefore, unless there is a breech in the skin, a pathogenic organism residing in the skin does not have the ability to enter the body through penetration.
Mucous membranes as a barrier to infective organisms and irritants
Apart from the skin, the second type of surface that is directly in contact with the outside air is the mucous membranes. These membranes line the passages such as the oral cavity, nasal cavity, respiratory passages, vagina, rectum, esophagus, urinary tract…etc. The mucous membranes are generally moist and therefore are permeable to the organisms and other irritants. However, the fluids that keep the mucous membranes moist such as saliva, tears and mucus acts against these irritants. The flow of these fluids can exert a wiping effect on the irritants and the tiny protrusions present on top of some of the mucous membrane lining cells (cilia) would push the irritants towards an ‘exit’, thus propelling them away from the body passages. Similarly, the hair present in the nasal mucosa can even trap larger particles without allowing them to enter the respiratory passages.
Non-specific chemical defenses
The non-specific chemical defenses in the human body can be seen in relation to most ‘inlets’ and ‘outlets’ of the body. For instance, the saliva and the tears would contain the enzyme ‘lysozyme’ and this has the ability to break the bacterial cell wall and thereby destroy the pathogenic bacteria trying to invade the human body. The acidic content in the stomach on the other hand acts as a barrier for pathogenic organisms and other irritants by being a corrosive agent, which can dissolve and destroy such invaders before they reach the other parts of the intestine. Similarly, the lubrication present in the vagina is also acidic, because it contains lactic acid and would act on the pathogens trying to invade through the vagina. It is also known that, the male urethra is kept pathogen free by the presence of bactericidal ingredients in the semen.
Fever as a non-specific defense mechanism
Although considered distressing, fever itself can be considered a non-specific defense mechanism. By raising the body temperature, it is possible to inhibit the growth of the heat sensitive pathogenic organisms within the body and stimulate the immune system. However, when the fever rises beyond a certain point, it could pose a danger to the body by itself and therefore modern medicine take into account the beneficial effects of suppressing the fever when dealing with infections along with the relief that it brings to the patient.
Inflammation is another non-specific defense mechanism against damages caused to body cells. Once damaged, a cell can release a substance known as histamine and this can initiate a cascade of events that would increase circulation towards the affected area and promote white blood cell mediated defenses. Similarly, a chemical compound known as interferon would also function as an anti-viral agent when viruses attack the cells.
The function of phagocytes and macrophages
Phagocytes and macrophages are two types of cells, which have the ability to engulf and clean the circulation and tissue structures from pathogenic organisms and other foreign substances including damaged cells. These cells circulate throughout the body and in certain instances may concentrate more at a particular location because of its higher risk of invasive attacks or because of the inflammatory responses as described earlier.
Thus, it is apparent that the body contains many barriers and non-specific defense mechanisms to counteract the attacks made by pathogenic organisms and other substances. However, it is only when these mechanisms fail that the body has to rely on the more specific immune responses to fend off any damaging effects.