Have you ever been around sick people and not gotten ill? We know that sickness spreads really easily between people, however a lot of times we are protected from it by our own bodies. While the human body needs to be a comfortable environment for the cells that comprise it, it also needs to fend off many other organisms that would like to take advantage and hijack our bodies for their own benefit. We call these harmful organisms pathogens and the body must protect itself against these. It may do this in a specific or non-specific manner. This article will explore the non-specific defence methods the body employs.

What are the Non-Specific Defenses of the Human Body?

The defences employed by the body to protect itself against foreign organisms may be either specific or non-specific. Specific defences do what they say on the tin, specifically targeting a particular pathogen.

Non-specific defences, on the other hand, do not bother to identify exactly what they are defending against. Instead, they focus on either preventing organisms from gaining entry to the body, removing them once they have entered, or blindly attacking anything they identify as foreign. Non-specific defences occur throughout many layers within the body, beginning with the skin, and working inwards. It is these layers that this article will explore.

Examples of Non-Specific Defenses of the Human Body

You might think that your body does little to protect you from infection before you get sick, but that is absolutely not true. There are many organs in our bodies that have a defensive function among other things. We will explore a few of these organs and see how they contribute to keeping us healthy.

The Skin as a Non-Specific Defense System

The skin is the human body's largest organ, forming a tough outer barrier. This not only keeps all of our component bits and bobs in the right place inside us but also keeps foreign material and organisms out. The skin is formed from tightly bound together epithelial cells, topped off with a layer of dead cells, forming a tough, nigh impenetrable barrier.

However, this does not mean that the skin is impervious to foreign organisms, as they may still be able to take hold in intact skin, or the skin may be damaged, creating an entry point.

Chemical Defences of the Skin

The skin also secretes a number of antimicrobial compounds, known as antimicrobial peptides and lipids. These prevent infection by cleaving the membrane of bacterial cells, thereby killing them and forming a structural part of the skin's defensive layer.

Another feature of the skin that acts as a defence against foreign pathogens is the skin's pH. Fatty acids, lactic and trans-urocanic acids are secreted onto the skin, maintaining the pH at 5.4-5.9. This pH makes the skin an inhospitable environment for foreign organisms. The pH disparity between the skin and the blood's pH of 7.4 provides another chemical layer of defence.

The Airways as a Non-Specific Defense System

Any area in which foreign substances enter the body is also a possible way foreign organisms may enter the body. This includes the respiratory tract.

The Nose

The inside of the nose, like the rest of the airways, is lined with mucus. This thick, viscous fluid traps debris and foreign organisms, preventing them from progressing deeper into the body. The nose is also lined with hairs, which, combined with the mucus, strengthen the physical barrier against entry into the body. When you then blow your nose, or it runs, the debris is removed from the body.

The Trachea and Bronchi

These are the tubes that conduct air from the outside environment down into the alveoli of the lungs. Like the nose, they contain mucus along with hair-like projections. The mucus is produced by cells known as goblet cells, which secrete mucus into the airways.

The hair-like projections of the trachea and bronchi are significantly smaller than the hairs found within the nose. They are known as cilia, and the cells from which they grow as ciliated cells. These hairs gradually waft mucus up to the top of the airway, where it eventually is either removed from the body or swallowed into the stomach.

The Stomach as a Non-Specific Defense System

As you may know, the stomach is lined with cells known as parietal cells, which secrete concentrated hydrochloric acid. This acid chemically destroys the pathogens that enter the body.

Cellular Non-Specific Defence Mechanisms

Apart from generic physical and chemical barriers, the human body has a series of specialised cells that are part of the non-specific defence system. These are part of the innate immune system, the portion of the immune system which presents a non-specific defence against foreign bodies and organisms. This is mediated both by chemical processes and cells.

Inflammation and fever

Inflammation is a natural immune defence process characterized by redness, heat, swelling, pain, and sometimes loss of function in the affected area. Inflammation is initiated by the release of chemical signals, such as cytokines, which attract immune cells to the site of injury or infection.

Inflammation happens because the damaged cells activate and release chemical signals that attract cells from the immune system, like neutrophils. On top of that, the chemical signals allow fluid to escape the blood vessels so that those cells can more easily enter the damaged area. That is why swelling happens during inflammation, and why some people can experience pain if the fluid build-up presses on their nerve endings.

Fever is also a generic defence mechanism to help your body fight off infection which consists in elevating body temperature. Bacteria and viruses that infect humans thrive at a certain temperature, mostly the average 37ºC of the human body. When their host gets hotter, they struggle to survive and reproduce as effectively. The extra heat also helps activate the immune system.

Phagocytosis

Phagocytosis is the process by which specialised cells of the body engulf and destroy pathogens and other small foreign particles. During phagocytosis, the foreign body is wrapped up by the phagocytosing cell's plasma membrane, forming a type of vacuole known as a phagosome. Once the phagosome is floating within the cytoplasm of the cell, the foreign body is digested and destroyed.

This destruction occurs through the fusion of granules containing enzymes, antibiotic proteins and many reactive oxygen species with the phagosome.

Non-Specific Immune Cells

The body contains a number of cells which have specifically evolved to counteract the presence of foreign bodies and organisms. Phagocytes are cells whose whole function is to destroy pathogens through phagocytosis. Their surface is covered in receptors, which recognise the presence of a foreign object or organism and trigger phagocytosis. There are three main subtypes of phagocytes; neutrophils, macrophages and dendritic cells.

Specific vs Non-Specific Defence Mechanisms

Specific and non-specific defence mechanisms work in parallel to avoid or eliminate infections in the human body. Even though they are team mates, they are quite different systems that complement each other.

Hopefully, you now understand why you don't get ill every time you come across a sick person. Remember, non-specific defences do what they say on the tin, they are non-specific and don't target any particular pathogen.