The Immune System


Our pets’ immune system functions in much the same way as our own, helping to protect the body from infection and illness. We tend to take it for granted much of the time, unless there is illness or it does not work properly. But what actually comprises our immune system? How does it work to protect the body from disease and what happens if it is not functioning correctly? This fact sheet looks at all these topics and provides some information on how can we support and naturally boost our pets’ immune system when it’s not working as it should.


The fundamental function of the immune system is to act as the body’s main defence mechanism, protecting it from all sorts of potential invaders and yet being able to distinguish between these and the body’s own tissues. It is also responsible for identifying and eliminating cancer cells.

The immune system is quite sophisticated and works hard to protect us from bacteria, viruses, fungi and parasites, as well as foreign substances such as dust, dirt, debris, pollens, toxins and the like, all of which could potentially cause ill health or disease. This protective, policing role is carried out largely by the white blood cells, which are produced in the bone marrow and which circulate in the blood. However, they are not actually the first line of defence, as the body has some formidable barriers to cross first. These are:

  • The skin, which is a waterproof protective barrier against invading organisms, dirt and debris.
  • The stomach, which contains acid to kill off bacteria and viruses. The natural peristaltic motion of the gut, as well as bile and digestive enzymes, also play a role in the protection of the gut from organisms. The stomach acid of dogs and cats is far stronger than our own, which is why they can eat food that would upset our digestive systems and make us quite ill.
  • The lining of the respiratory tract, which produces mucus to trap and remove organisms and particulate debris, by coughing or sneezing.
  • The lining of the gut, which has a very active protection system, as well as a protective mucus layer.
  • Saliva and tears, which contain antibacterial enzymes.
  • The liver, which can filter out material that has gained access to the blood stream and which can also remove toxins and poisons.

If an organism or foreign material gets past these hurdles, then the immune system springs into action.


There are several different ways in which the immune system can respond to foreign invaders.

The first involves a mechanism known as the Innate immune response, which comes into play quickly and efficiently to remove invading microorganisms or toxins. This mechanism is generic in its action, rather than being specific, and leaves no immunological memory or protective future immunity. It has two basic functional components:

1.  Cell mediated immunity. This involves the activation of various types of white blood cells and does not involve antibodies. The main cells involved in the innate response are:

  • Natural killer cells (NK cells) and macrophages, which engulf and destroy pathogens.
  • Neutrophils, which are the most abundant of all the white cells and which contain substances toxic to bacteria and fungi.
  • T-lymphocytes, which destroy cells that are dysfunctional, damaged or infected by viruses or bacteria.
  • Mast cells. These release histamine, which causes the characteristic reddening of inflamed tissues and increased blood flow, which then brings in other white cells to the area. Mast cells are also involved with allergy problems and in healing wounds.
  • The release of chemicals called cytokines, in response to the foreign material or antigens, which activate other cells involved in the immune response.

 2.  Humoral mediated immunity. This involves immunity which is based on substances found in body fluids (the “humours”) and is principally based around the production and action of antibodies:

  • Antibodies are produced by cells known as B lymphocytes, which transform into plasma cells which then secrete antibodies. The antibodies then bind to antigens (foreign material) on the surface of invading microorganisms (such as bacteria, viruses or parasites), signalling or highlighting them for destruction.
  • In addition, the Innate humoral response is backed by a biochemical cascade system called the complement system, which assists the destruction of pathogens by antibodies.

The second and most important type of immune response is known as the Adaptive Immune Response or The Specific Immune System. This type of response also has both cell mediated and humoral components. It is slow to come into play, but is particular to the invading organism and involves specific “signature” antibody production. The immune response is remembered, so if the organism is encountered again, the immune system is primed ready for action. The ability to produce this tailored type of response is maintained in the body by specific memory cells.


The signs of inflammation are very familiar, encompassing redness, swelling, heat, fever, increased blood flow to the affected area and pain, all of which can be triggered by injury or by infection. It is one of the initial fundamental responses of the body’s immune system to being challenged. The process is initiated by a complex cascade of chemical interactions involving substances released from infected or damaged cells. These include prostaglandins that cause the signs associated with a fever, leukotrienes, which attract white blood cells to the area and cytokines, which allow communication between cells, as well as attracting cells to the area. Some cytokines, such as interferon, also have anti-viral effects.

Problems with the immune system

Inflammation can be useful, but can also be a huge problem. Invasion by a virus (such as the flu virus in cats) can initiate an immune response that may result in a fever and signs of illness. However the immune response should ultimately eliminate the virus quickly, after which the body recovers. However unwanted chronic inflammation of a joint, as in many cases of arthritis, can be very painful and may cause chronic lameness. Overreaction of the immune system can result in symptoms ranging from an irritating skin allergy to serious, life threatening autoimmune conditions.

Problems with the immune system include:

Underperformance. This means that the immune system is not able to deal with challenges. This is sometimes termed immunodeficiency and can result from the patient being:

  • Young or elderly, where the immune system functions less effectively.
  • Overweight, as obesity affects immune system function.
  • In a poor state of health or run down.
  • Malnourished.
  • Infected by immunosuppressive viruses such as Feline leukaemia virus (FeLV) or Feline immunodeficiency virus (FIV) in cats for example.
  • Treated with immunosuppressive drugs such as steroids or cyclosporins.

In contrast the immune system can overreact inappropriately leading to a range ofautoimmune conditions where the immune system attacks the body’s own tissues as if they were foreign.

Autoimmune diseases include rheumatoid arthritis, diabetes mellitus type 1, Inflammatory bowel disease (IBD), Autoimmune haemolytic anaemia (AHA), Immune mediated thrombocytopaenia (IMT), Pemphigus and Systemic lupus erythematosus (SLE).

The immune system can also react in another way, leading to hypersensitivity. There are 4 classes of hypersensitivity based on the exact mechanisms involved and on the length of time the immune system takes to react. One of these is the mechanism which results in the symptoms linked with allergies such as hayfever in humans and in some forms of allergic eczema/dermatitis in cats and dogs.

The immune system can also cause health problems linked with long term or chronicinflammation. Illnesses of this type are common. The pain associated with arthritis is a good example, as is chronic bronchitis, inflammation of the upper airways.

Treating and supporting the immune system

Conventional medicine is largely concerned with altering or modifying the immune response, usually by toning it down. Immunosuppressive drugs such as steroids and cyclosporins are frequently used to treat skin allergies and autoimmune conditions. Chronic inflammation, particularly where pain is involved and fevers, is often treated with non-steroidal anti-inflammatory drugs (NSAIDS), which modify the inflammatory response. Many of the drugs that modify the immune system work through blocking the action of prostaglandins and are known as prostaglandin antagonists.

Alternative medicine however, is more concerned with supporting, strengthening or modulating the immune system, taking in effect a holistic view and utilising a variety of means to help:

  • By using diet. A good balanced diet is essential, particularly one which is free from artificial chemicals such as preservatives and colourings, both of which can interfere with the way in which the immune system works.
  • By reducing obesity. With many overweight pets, reducing obesity by increasing exercise and feeding correctly provides good background support for the immune system.
  • Adding in quality supplements. Basic supplementation with vitamins (particularly vitamin C and other natural antioxidants), mineral and trace elements is essential if the diet is poor, highly processed or if it contains chemical additives.
  • Using Nutraceuticals. These more specialised supplements are designed to play more precise roles and include supplements which can assist the immune system. This category incorporates derivatives from colostrum such as Transfer Factors as well as Beta glucans, Inositol and Phytosterols.
  • Using Herbal Medicine. A number of herbs can stimulate and support the immune system. Some of the best known include Echinacea, Sage, Elderberry, Garlic and Aloe vera.
  • Using Chinese Mushrooms such as Maitake or Shiitake mushrooms, which strengthen the immune system.
  • Using probiotics. The relationship between the health of the gut and the immune system is well known. Probiotics can be used to restore and stabilise the natural bacterial flora of the gut, thereby supporting the immune system.
  • Using Acupuncture. This form of treatment based on placing needles at specific points on lines of energy called channels or meridians, can help support the immune system.

How Denes can help

Denes has a number of products that can help support your pet’s immune system:

Other useful Denes Factsheets