|But not Donkeys!|
It's both a physical and a biological entity and it's responsible for a lot of medicine, from vaccinations to blood transfusions and organ transplantation. It's also involved in diseases such as auto immune disorders which are increasingly common.
Basically, immunity is this amazing system that keeps most of us healthy. It can be summarised as
Parts of the Immune System
This is the cell's innate ability to detect foreign material and eliminate them without the bodies own specific response. It is a generic quick response to disease causing organisms (Pathogens) which invade our body to either prevent an infection or slow it doen sufficiently for the slower but more effective and selective Adaptive Immune system to activate. It consists of
Anatomical barriers -
These consist of physical barriers. The skin itself is impermeable to pathogens providing defences like a solid wall. Our nasal passage is lined with mucous that is constantly moved into the stomach catching pathogens and killing them. Our eyes are covered in caustic tears and our mouths in saliva which contains a variety of enzymes. all these ensure that the vast majority of pathogens are killed before they can even enter an area where they can cause harm.
This is not a bad thing but the symptoms we associate with inflammation are due to the localised response to the presence of a foreign body or pathogen. It's main purpose is to provide a physical barrier to control the spread of infection and to heal damaged tissue in the region.
Damaged cells give out a bunch of chemical factors which cause pain and blood vessels to become more permeable. This attracts phagocytes and alerts you to the injury allowing you to cover it to prevent further infections. Phagocytes are "cells that eat", they recognise foreign or dead tissue and physically consume it and digest it.
The inflammation is simply noted by the moniker of rubor, calor, tumor et dolor (red, hot, swollen and painful). Normally inflammation is a good thing (it's what makes insect bites itch) but its bad in auto immune diseases such as arthritis.
Complement System -
This is a biochemical cascade that once activated causes the influx of phagocytic cells from blood into the tissue. The thing to remember is that our blood vessels are actually permeable and the phagocytes can actively squeeze through to get into the tissue area.
It is via this system foreign cells are recognised and either "eaten and digested" by the White Blood Cells or are lysed by general antibodies. The complement pathway has a long evolutionary history with parts of it even seen in plants.
Cells Involved -
The white blood cells (WBC) involved are
Mast Cells - A cell associated with allergy and anaphylaxis. A mediator of inflammation response.
|Mast cells, the pink dots|
are histamine which causes hayfever.
Phagocytes - Large cells that move like amoeba. They "eat" other cells by surrounding them with their plasma membranes producing "bubbles" in which they can release enzymes safely without damaging other cells. They have a role in normal development as our normal cells do "commit suicide" in specific ways to produce our shape. These help with clean up.
Macrophages - Large Phagocytic cells that efficiently consume multiple pathogens. Heavily motile and actively cross from the blood stream into tissue to hunt down pathogens. They kill by the use of free radical oxygen.
|Macrophage "om nom noming"|
Neutr/Eosin/Basophils - A series of similar cells that target a variety of different pathogens non specifically. Have "granules" similar to ground coffee that have specific actions.
|The big pink cell with all the weird|
purple nucleus is a eosinophil
Natural Killer Cells - These are interesting, they don't target pathogens but cells infected by viruses or cancerous cells. They recognise these and induce controlled cell death halting the spread of cancerous tissue or viruses.
Dendritic Cells and Gamma/Delta T Cells - These are the bridge between the innate and adaptive systems and their main role is antigen presentation. They harvest proteins from damaged pathogens and present them to T-Cells.
The dendritic cells activate our adaptive immune system. It's a series of specialised cells that produce antibodies which "mark" out pathogens for destruction with greater specificity. It basically any cell marked with an antibody gets destroyed. Evolutionarily speaking it's quite elegant utilising mutations (somatic hypermutation) to produce endless varieties of "antigen receptors". When an antigen matches a receptor, antibodies are produced specific to that antigen.
The antibodies are the main weapon of our body to fight disease. It is a larger response than innate immunity and once sensitised the adaptive immune system often fights of diseases even before we can manifest the signs of disease.
Cells involved -
These are called Lymphocytes mainly as they are found circulating in lymph more than in blood, their role is determined by the cell itself.
T - Lymphocytes - The main role of the cell is to recognise cells infected by viruses and trigger the apoptotic pathway destroying the cell and its viral load. Since viruses only replicate inside cells this kills them enmasse as phagocytes consume the destroyed cell debris and digest the contents. The antigen of the viral cell is recognised by surface antibodies on the T-Lymphocyte. There are also helper T-Lymphocytes whose role is control and organisation of the response.
B - Lymphocytes - The main role of these is to produce humoral (free floating) antibodies that recognise pathogens and mark them for destruction by activating the complement system and by causing the pathogen to become "sticky" but only with other pathogens. This causes them to clump together and make them easier to kill.
Memory Cells - After the infection has passed, most of these cells die. However a few are maintained in circulation as "memory cells". In future infections these are rapidly activated to produce a humoral response which quickly destroys any new infections even before they produce any symptoms.In Infants, there is a version of this system called passive immunity. Certain classes of antibodies can cross the placental barrier and can be absorbed through milk particularly the colostrum (first milk). These give the new born baby the same range of protection as the mother.
This is how our natural system works. It doesn't recognise our own cells as self recognising antibodies are actively destroyed. However in cases of organ transplant, blood donation or foetal blood incompatibility the body can target itself. In autoimmune diseases we can see a pathological breakdown of this system targeting the self rather than pathogens.
Hacking the System -
All of this sounds pretty damn awesome, but it's not infallible. Remember the pathogens themselves have their own tricks to avoid being caught and killed while remaining long enough to survive and spread. The range of tricks is colossal from lysogeny (the ability of a virus to integrate it's DNA into ours. Herpes does this! The common cold sore is a form of this ability.) to shuffling antigens (the common cold does this.)
So what we need is a way of beating diseases at their own game. A way of acquiring immunity to diseases without the disease itself.
That way is vaccination. The idea is simple, you take known antigens and inject them into the body. The body's cells recognise these "bits and pieces" and produce an immune response as if there were a real threat maintaining a population of "memory cells" to deal with subsequent infections. Every infection you stop actually increases your immune response. So there are a variety of different responses. The response to just an antigen mix is less than a dead/killed cell which is less than a severely weakened cell. However as long as we are under a vaccinated period we should be able to fight off an infection. It isn't 100% fool proof but it means that the vast majority of people are immune to a particular disease preventing it's spread in general.
Vaccination is merely a trick, to trick our bodies into thinking that it has fought of a disease when in reality it has not. At worst you save time and money that would have been wasted in treating the sick and at best you save lives out of it.