Non-specific Immunity

Over the past three blogs I’ve briefly spoken about the things that can make us sick and some of the issues arising from our treatment of these diseases. This essay is going to briefly look at the generic types of defences the human body has against invading pathogens. Next week I’ll start looking at specific defences and responses to pathogens. In vertebrates, non-specific immunity includes physical barriers, physiological mechanisms, chemical mechanisms, phagocytes and inflammation.

First line of defence

Barriers, physical and chemical

The first line of defence our body has against the hoard of invading pathogens is barriers. Our major barrier is the skin. The immediate layer of cells exposed to the external environment are dead. Pathogens need to get past these dead cells to the living cells beneath. This provides a physical obstruction and prevents pathogens entering our bodies. On the surface of our skin are many, many naturally occurring, beneficial bacteria. These also help by occupying all the available space on you skin’s surface so when a nasty little pathogenic microbe lands on our skin it is out-competed by the good guys. All available resources, including space, are taken. These natural non-pathogenic bacteria are also found in the gut and vagina as well as on the external surface. When a cut becomes infected, this is because there is a break in the skin barrier allowing a pathogen to enter. Our skin is generally acidic and has a coating of oils, lipids, that help create an environment unsuitable to many potential pathogens.

Other barriers include the mucus lining of your nose and throat, lungs and gut and this provide an obstacle to pathogens inhaled or ingested. Another powerful barrier is stomach acid, which destroys many pathogenic organisms you may have swallowed. Chemical barriers also exist. Your tears contain the enzyme lysozyme, which is also found in sweat and in many of your body’s tissues. This enzyme causes lysis in invading pathogens, destroying them.

Second Line of Defence

When we talk about second line of defence it is not a list of things that happen in a certain order. These usually happen at the same time and often one response makes another more effective.

Fever

Despite our best efforts, a pathogen manages to get past our barriers. There are then a series of non-specific responses our bodies go through to try and fight off disease. Fever is one of these. In response to the pathogen, macrophages – which we shall learn about in a while – release chemicals that cause the hypothalamus to reset the body’s thermostat to an elevated temperature. This aids recovery by killing some pathogens, facilitating the death of infected cells and increasing the activity of some lymphocytes and phagocytes – which we will also learn about in a bit. Fever can also be triggered by toxic by products of some bacteria such as pyrogens, however, the effects of elevated temperature on the bacteria is similar.

Protein based responses

Let’s say that the pathogen that invaded you was viral and has invaded your cells in the manner described in previous blogs. Another non-specific response your body undertakes is the release of antiviral proteins called Interferons. Interferons, as the name suggests interferes with stuff. In this case interferons interfere with virus replication and stimulate macrophages to come along and eat the infected cells. One other incredibly beneficial thing they do is they make non-infected cells more resistant to viral infection. Again, we’ll talk about this in a future blog.

I’m going to stick with proteins and talk about Complimentary Proteins. There are about twenty of these originally synthesised in your liver and are found floating around in your blood in an inert state. They become active when they come into direct contact with a pathogen. Another trigger for their activation is when antibodies and antigens combine (future blog). They defend us against pathogens in four main ways. They cause cell lysis in pathogens – our immune system really likes lysis as a response ; they can coat the pathogen which makes it easier for phagocytes to recognise and engulf them; they attract a type of cell called leukocytes to the area of infection to attack the pathogen; and, finally, they trigger the release of a chemical called histamine, which we will soon get to, and is involved in the inflammatory response. Complimentary Proteins are also involved in Specific Immunity and will be discussed later.

Cellular components: white blood cells and stuff

White blood cells, or phagocytic leucocytes, are a group of cells that engulf, ingest and destroy invading pathogens. They eat them!  Before we go any further, watch this: Phagocytosis

I’m going to talk about the cellular components of our non-specific immune response before returning to some more physiological and chemical responses.  I’ve mentioned phagocytes, macrophages and leukocytes so far. There are many more including: Lymphocytes, Basophils, Neutrophils,  Mast Cells & Natural Killer (NK) Cells. I’ll write a short essay about all these dudes plus a few more later. What you need to know now is that phagocytic cells originate in bone marrow and destroy pathogenic cells by eating them. I will expand on this in future essays. Pinky promise.

Inflammatory Response

Inflammation is when there is swelling and usually a localised temperature rise in an area of infection. The term for this is edema. This happens because enzymes and hormones such as serotonin are released as a response to a pathogen (I keep saying this but it will be in the next essay!). These compounds cause the arterioles to dilate and become ‘leaky’. There is increased blood flow to the site of infection transporting a greater number of defensive cells and compounds and as these arterioles become permeable (leaky), these cells and compounds move out of the blood and into the interstitial fluid to the site of infection where they attack the pathogen.

One of the compounds that cause the permeability and dilation of the blood vessels is histamine. We will look at where this comes from and exactly what it does in the next essay. (I’m building this next essay up. Hope it’s good!)
Inflammatory Response 

A quick recap

Our first line of defence is barriers, both physical and chemical. If these barriers are breached we have a second line of defence. This creates a hostile environment for the pathogen (fever) causes the release of chemicals that inhibit the pathogen’s functioning and cause phagocytic cells to eat them. The second line of defence also changes conditions within the body to increase blood flow to the site of infection, which in turn brings defensive cells and chemicals to the area.

The next blog will cover: the cells involved in our non-specific immunity including platelets and the role and production of histamine. I’ll put it all together and discuss our response to a pathogen infecting a small cut.

Until then, here is some Dave Graney for you