Friday, 4 September 2015

All About - The Immune System

In my Circulatory System blog I mentioned White Blood Cells (WBC's) so I thought it would be useful to understand more about how these and how the whole immune system works.  This system is responsible for resisting disease. It recognises and deals with thousands of different substances that enter our horses' bodies ...  saving them from death.  

Diseases are illnesses resulting from infection.  Infection is the invasion of the horse's body by an infectious agent.  These include bacteria, viruses, nematodes (eg: roundworms), arthropods (eg: lice), fungi (eg: ringworm) and others such as tapeworm.  These infectious agents also produce toxins.   The horse's body will react to these invaders and these reactions are the symptoms we see.  

The invaders (or infectious agents and toxins) enter the body and multiply until there are millions of them in the horse.  This is when the symptoms of the disease appear.  The time between infection and the development of the symptoms is the 'incubation' period, this can vary from a few hours to many months! 
How do the invaders enter the body?

The infectious agents can be swallowed (a poison) or inhaled, they can enter through the skin (a wound), the eyes, the urinary tract or via the genitals.  The skin is the first line of defence, look out for a future blog about the skin.  If the skin is breached or the invaders enter through a different route the White Blood Cells are the second line of defence.

What happens next?

Some infectious agents will invade the whole body and some will stay in one part.  They vary in their ability to invade, multiply and exist - this is called their virulence.  Bacteria with a high virulence will invade the blood stream and cause blood poisoning (septicaemia).  Bacteria with a low virulence will just cause a local infection eg: an abscess.  Some bacteria and viruses produce toxins which will spread through the body. 

Acute diseases develop rapidly and last a short time and generally mean that the horse's defences are inadequate.  Chronic diseases are usually more slow to develop and last longer and happen when the horse's defence is unable to overcome the disease but is able to 'hold it'.

How does the horse fight back?

The horse's second line of defence is the immune system.  The immune system uses White Blood Cells (WBC's) to fight the invaders.  They do this by attacking and destroying the germs.  WBC's are also known as leucocytes. 

WBC's are divided into 2 categories granulocytes (or polymorph's) and agranulocytes.  

Agranulocytes originate in the lymphatic system and are subdivided into lymphocytes and monocytes.  

Polymorphs originate in the bone marrow like the Red Blood Cell's and can be subdivided into neutrophils, eosinophils and basophils.  The neutrophils engulf bacteria to form pus.  The eosinophils detoxify foreign proteins and the basophils help control inflammation

There are 2 components to the immune system; the non-specific immune system and the specific immune system.  

1. The non-specific immune system is present from birth and is effective against many different pathogens (something that can cause disease) and foreign substances.  It is a quick response system but always gives the same response. If skin is irritated by a foreign substance, like a horse fly bite, mast cells release histamine which causes inflammation - pain, heat and swelling.  

2. The specific immune system attacks, disarms, destroys and removes foreign bodies.  It is a slower response system.  However, this system can create an immunological memory which means that on repeated infection with the same pathogen the response is quicker!  The specific immune system is able to distinguish between friend and foe and will only attack antigens from foreign organisms and substances.  Antigens are molecules on the surface of every cell membrane.  

There are 2 main types of response by the specific immune system:

a. The Cell-Mediated Response:-  Cells specific to the antigens, pathogens or foreign substances are produced.  These lymphocytes are called T cells.  One type of T cells activate other cells in the immune system.  

Macrophages are large phagocytes which destroy pathogens.  These are derived from monocytes. 

Killer T cells attack altered body cells (eg: cells with viruses) and large pathogens.  These T cells punch holes in their opponents!

b. The Humoral Response: - Anti-bodies are specific to a particular antigen.  The anti-bodies are produced by small lymphocytes  called B cells.   These B cells mature in the bone marrow and there are a great variety of them, which means it is likely that at least one will match a particular antigen.    These B cells also produce the memory cells which will improve the response time if a second attack occurs by the same antigen.  These are central to the workings of vaccinations.  


If a horse has suffered from a disease it's body will develop the specific WBC's needed to fight that disease and so has become immune to the disease.  This immunity can last for a few weeks or many years.  

For many diseases vaccinations have been developed using both dead and live (but weakened) micro-organisms.  These introduce the bacteria to the horse's body, the immune system produces the relevant WBC's to fight and also produces memory cells.  Thus if the horse encounters the micro-organisms again it will have the ability to respond more quickly.

As mentioned above, the duration of immunity varies and so some vaccinations must be redone each year eg: flu and some every 2 years eg: Tetanus.

Other ways to control bacteria.

Some bacteria can be killed using disinfectants or antiseptics.  Disinfectants are strong and aggressive and so should not be used on a horse.  These are useful to destroy the micro-organisms which may be found on equipment or stables - as with a Strangles outbreak all equipment, clothing etcetera,  etcetera  is thoroughly disinfected.  Antiseptics may only stop the bacteria from multiplying but can be used on a horse and on wounds safely.  

Antibiotics are chemical substances which have been developed to stop or kill bacteria.  The dosage and length of time these are given is important to ensure the bacteria do not become resistant. 

Are all bacteria bad for the horse?

Bacteria are micro-organisms.  Some bacteria are good, there are many thousands living in the horse's gut breaking down the food (see my Digestive System blog).  However, some are capable of causing  disease, these are pathogenic. 

Worms (nematodes, tapeworm etcetera) are controlled with paddock management and the use of chemicals - see my Worming blog.  Lice and ringworm are controlled with topical powders or washes.  Viruses are difficult to control and are not affected by antibiotics. 

It is amazing what the Immune System can do and understanding a little how it works can help us give it the best chance possible to work efficiently.

Have you read my other blogs about Horse Health?
The Healthy Horse
Recognising Illhealth
Minor Wounds and Nursing a Sick Horse
First Aid for Horses

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Until next time!

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