The immune system isnt just one single thing in one place. There are many facets to this complex system, and many elements in place that can protect our body from opportunistic pathogens and and damaging infectious agents. Each aspect of immunity will offer different types and different levels of protection.

This is the second level of immunity. This is the first aspect of immunity that kicks in, should a pathogen of some sort penetrate the physical barriers. It is called our innate immunity, because we are born with it fully functioning, ready to do its thing as an when needed, unlike other aspects of the immune system that develop over time, as you will see a little later. Within our tissues, are several types of white blood cell that circulate the body looking for any kind of foreign invader or damaged tissue. One of the main types of cell within the innate immune system is the macrophage. These large white cells engulf pathogens, and send chemical signals to other circulating immune cells, to localised circulation, and to nerve endings. This will all be discussed in greater detail once we start to look at each different blood cell in turn. There are also cells such as natural killer cells and toxic biochemical compounds that form part of innate immunity too, but again, this will all begin to come clear when we get up close and personal with the different cells of the immune system.

Adaptive/Learned Immunity

This is the third stage of immunity, and one that is unique to only a handful of species of animal on the planet. Most animals seem to cope just fine with innate immunity. Human beings have developed this third powerful arm of the immune system, for reasons unknown. The adaptive immunity, is the type of immunity that is antibody mediated. This basically means that the immune system, once challenged by a specific pathogen, learns the correct way in which to respond to it in future. This branch of the immune system is actually able to manufacture specific specialised proteins which circulate, and are able to tell the immune system what to do, should the body be invaded by the same pathogen in the future.

Macrophages

Macrophages are a very large white blood cell whose primary role in life is eating! Macrophage actually means ‘large eater’. They are found in high numbers in our tissues, where most of the time they are just in a resting state, milling around, swallowing up general junk and waste such as metabolic by products and waste materials that have been kicked out of cells. If our first line of defences, ie the physical barriers become breached - such as if we get a cut or a splinter etc, then macrophages begin to receive a variety of chemical signals. This chemical cascade alerts the macrophage and changes it from a resting state to an active state.

Once activated, the macrophage continues its eating habits, but to a much more aggressive extent. They approach invading pathogens and begin to elongate themselves. Rather than waiting until they bump in to the pathogen, they stretch themselves out to reach it as soon as possible. Once they reach it, they begin to engulf it and surround it, forming like a pouch, called a vesicle. This pouch is then drawn inside the macrophage, where it floats around like a little bubble containing the pathogen. Inside the macrophages are other bubble like things called lysosomes. These bubbles are filled with extremely potent chemicals and enzymes that can completely destroy bacteria. The lysosome binds with the vesicle, exposing the bacteria to the toxic substances, destroying it completely. Once the bacteria has been destroyed, its remnants are then spat out again for removal. This isn't the only way that macrophages help with our immunity. They also can deliver messages to other cells of the immune system. When they become activated, they display something called ‘major histocompatability complex class II’ (MHC II for short) on their outer surface. They incorporate fragments of proteins from the invader into the MHC II and display them to cells called T Helper cells. Once the T Helper cells have seen this signal, they can then go about instigating further relevant immunological reactions such as localised inflammation.

Neutrophils

Neutrophils are the most abundant white blood cell in the body. They make up up to 70% of the entire white cell population of the body, and we produce up to 100 billion of these cells per day! They are also a phagocytic cell (meaning it engulfs things like a macrophage does). They do not act as antigen presenting cells like macrophages can (remember how macrophages display MHC to other cells of the immune system), they are purely professional eating machines. These potent cells dont just eat and destroy willy nilly, they are actually moving through our circulation in a resting state. it is only when they leave the circulation that they become activated. How they get activated and do their thing is truly fascinating. They are moving through the vessels in our circulation at an incredibly high speed. When macrophages make the first initial contact with a pathogen, as we have seen one of their responses is to secrete a group of compounds called cytokines - chemical messengers. When relevant cytokines are secreted by macrophages, they stimulate the production of a protein called selectin. This protein then gets shuttled to the endothelium - the inner lining of the blood vessels that we discussed in great detail in the heart module. When this protein is displayed on the endothelium, it binds to an adhesion molecule on the surface of neutrophils, called selectin ligand. When this binding takes place, the effect is a bit like velcro. The neutrophils stick to this protein, which massively slows down their movement, and instead of being swept through the vessels at light speed, they instead slowly roll along the lining of the vessel. This drastic slowing then allows the neutrophil to ‘sniff out’ further cytokine signalling from macrophages, which tells it where the inflammatory battle is taking place. Chemicals called chemoattractants pull the neutrophil from the inside of the vessel, through the vessel wall, and in to the tissues, towards the affected area. As the neutrophil leaves the circulation, it is then activated, so it arrives ready for action. Much like macrophages, neutrophils are filled with lysosomes filled with incredibly highly toxic compounds that can rapidly and aggressively destroy pathogens that they engulf. They also engulf pathogens in a far faster and more aggressive manner than macrophages do. Once a neutrophil has delivered as much of its phagocytic function as it can, it then dies. When you have an infected wound, that gets filled with pus, this pus is composed predominantly of dead neutrophils!