The immune system of dogs and cats

The canine and feline immune system affects each and every system and cell in their body. It is a barrier of detection and elimination that stands narrowly between the healthy cells of the body and invading organisms like viruses, bacteria, parasites, cancer cells, pollen or food molecules. The immune system is the body’s defence against illness, disease, infection and allergies, and it has a few mechanisms of protection against pathogens that may attack a healthy animal.

In this article, we’ll look at the immune system of dogs and cats, unpack the organs of the immune system, and look at how it works. We’ll also explore immune-related illnesses and the symptoms they present.

How does the immune system work?

The immune system uses a multi-pronged defence against attack from the outside, harnessing multiple methods of detecting, defending, and eliminating harmful pathogens from the body. The immune system is always active and ‘en garde’ – ready to defend the cells, tissues and organs of body. The key to the immune system functioning properly is its ability identify good cells and substances (‘self’) and invasive cells and substances (‘non-self’). In a healthy dog or cat, the immune system attacks ‘non-self’ antigens, but sometimes the immune system launches an attack on ‘self’ cells because of mis-identifying healthy cells as antigens.

Antigens are any substances – viruses, bacteria, microorganisms, cancer cells, pollen or food molecules – that trigger an immune response. The immune response comprises recognising an antigen, gathering the B- and T-cells together to launch an attack, and then destroying the invading antigens. The lymphatic system plays a large role in the immune response, in that it is responsible for the production of antibodies and the transport of white blood cells and proteins to the parts of the body where they are needed against a pathogenic attack.

There are many layers to the immune system, all of which put up a strong defence against pathogens, especially if the dog or cat is healthy.

Types of immunity in dogs and cats

There are different types of immunological strategies to ensure the immune system covers all the bases of attack:

Innate immunity

Innate immunity (or non-specific immunity) is what dogs and cats are born with and which is built up by the mother’s colostrum and good nutrition. It is the body’s general defence against any antigens. Innate immunity is triggered by any pathogens attempting to gain access to the body. It consists of defences such as:

Anatomic defences

Anatomic defences include a range of barriers that prevent pathogens from invading the body, either through exclusion or extermination, such as:

Physical barriers: skin and mucosa, cornea, and membranes lining the digestive and reproductive tracts.
Mechanical defences: The mucociliary escalator moves mucus (containing pathogens) out of the upper respiratory tract via mucosal cilia; and peristalsis assists in the process of excreting faeces (and pathogens) out of the body. Coughing and sneezing also play an important role in quickly removing larger irritants from the respiratory tract.
Microbiome: The beneficial bacteria on the skin and in the gut are another layer of defence against harmful bacteria. Vomiting or diarrhoea are two immune responses that ensure rapid expulsion of antigens from the gut – the result of quick detection of harmful substances by the immune system.

Inflammation

Inflammation may look and feel unpleasant, but it is a key defence mechanism in innate immunity. It is triggered by the detection of ‘non-self’ substances invading the body and occurs when the immune system sends inflammatory cytokines to kill off the invading pathogens. Macrophages then step in to remove the invaders and cell debris and to repair damaged cells.

During this inflammatory response, the animal can experience fever, fatigue, sleepiness, and reduced appetite. This encourages rest and inaction so that the body can redirect energy that would have been used for digestion and movement, to fight off viruses and bacteria.

Chemical defences

The animal’s tissues contain protein peptides and enzymes that have an antimicrobial effect on invasive bacteria. This is called the complement system or complement cascade. It is very carefully regulated in the body because it can also damage healthy tissues.

The mechanism of innate immunity in the body is managed by a complex process involving specific cells that detect invasion, engage fighter cells and protein molecules, and trigger defence events in the body to ensure there are enough leukocytes (white blood cells) to kill off the invading antigens.

Adaptive immunity

While innate immunity offers a broad spectrum of defence against pathogens, there’s no guarantee that this type of immunity is enough to fend off serious viral, bacterial, fungal or parasitic infection. This is where adaptive immunity is crucial.

Adaptive immunity is acquired immunity – meaning it is built up after repeated exposure to specific antigens. With each exposure to various antigens, the immune system ‘learns’ how to attack and neutralise these threats with antibodies, launching a faster and more effective defence at each exposure. It also responds proportionally to the microbial threat: a small viral load will result in only the necessary defence to neutralise it. A larger invasion will necessitate a stronger defence.

Adaptive immunity can be developed with vaccination. With each introduction of small amounts of a pathogen via a vaccine, the pet’s immune system ‘remembers’ how to attack it and produces the right type of antibodies to eliminate it.

A very good example of this is the rabies virus. Innate immunity is not enough for the body to defend itself against the infectious disease of rabies. However, when puppies and kittens receive their first rabies vaccine at three months old, it primes their immune system to attack the virus, should they encounter it through a bite or scratch from a rabid animal. When they get their second vaccine at 12 months, it triggers their immune system once again to ‘remember’ how to fight off the rabies virus. Booster vaccines at each subsequent one-year or three-year mark ensure that the dog or cat’s immune system maintains the ability to produce a sufficient level of antibodies to effectively attack the virus, if necessary.

Organs of the immune system

There are many organs in the body that are directly involved in the immune system’s defence process. The lymphatic system (and to a certain extent the cardiovascular system) is the transport system that essentially connects all the organs of the immune system together. Immune system organs include:

Lymph nodes

The lymph nodes are the primary sites in the body where immune responses are generated; where antigens are caught and neutralised. In dogs and cats, lymph nodes occur in the throat (tonsils), neck (submandibular), chest (prescapular), armpits (axillary), groin (inguinal), and behind the knees (popliteal). Lymph nodes are bean-shaped structures that swell and can be felt through the skin when they become enlarged because of illness in the near vicinity. For example, if a dog has a tooth infection, their submandibular lymph nodes will likely be swollen when the vet feels their neck.

Leukocytes and lymphocytes

A major part of the immune system are the leukocytes or white blood cells that are made in the bone marrow and circulated in the blood and in the lymphatic system. Lymphocytes are specific kinds of leukocytes, and consist of B cells and T cells, which recognise, attack and destroy invasive antigens. They produce antibodies to deal with the antigens, after which, these immune cells are eliminated. However, some of the antibodies are retained to ‘remember’ an antigen and help the body to launch a quicker, stronger response to the same antigen in future. B cells and T cells assist with adaptive immunity and are the part of the immune system targeted by pet vaccinations.

Spleen

The spleen plays a major role in pets’ immune systems. It is the organ that filters antigens from the blood, and generates lymphocytes to target and kill the invading antigens. It controls the level of red and white blood cells in the body to keep the immune system in balance and functioning properly.

Bone marrow

Pets’ bone marrow is the site of red and white blood cell production, as well as the production of blood platelets. When the immune system responds to invading antigens, the bone marrow releases the appropriate matured B cells to fight them off. It also replaces blood cells that have reached their complete lifespan or have been used up in fighting infection.

Thymus

White blood cells are also produced by the thymus gland. T cells are produced by bone marrow, but mature in the thymus before being released to fight infection. The thymus plays a significant role in establishing immunity in puppies and kittens, and is at its largest when they are very young. Its proportion shrinks as they grow and their immune system strengthens.

The skin

The skin is the largest immune organ of the canine and feline body. It creates a physical barrier between the outside world and the animal’s blood, organs and internal cells. When bacteria, viruses, fungi and other harmful microbes occur on the skin, it detects their status as antigens before launching an attack. The healthy bacteria on their skin help to keep their microbiome in balance. When there is trauma to the skin (graze, cut or other damage), the skin repairs quickly to prevent harmful pathogens from entering the body.

Immunity issues in dogs and cats

As with any system in pets’ bodies, the immune system can be affected by stressors, genetics, or the malfunctioning of any of the other systems. The immune system can be overactive, causing it to harm the body itself, or it can be underactive and not protect the animal as effectively as it should. These disorders are known as immune-mediated disorders, causing immunodeficiencies (underactive) or autoimmune disorders (overactive).

There are four types of disordered immune reactions in pets:

Type I: Allergic reactions (anaphylaxis)

When a dog or cat has been previously exposed to an antigen (such as pollen or a food protein) and encounters it again, their body may produce too many antibodies and overreact to the presence of the antigen. This over-stimulation of the immune response can result in a severe allergic reaction.

When an antibody attacks an antigen (invader), this action produces histamines and other chemicals, which have a direct influence on muscles, blood vessels and other organs and processes. Under normal circumstances, this response works to rid the body of the antigens by disabling, killing and eliminating them. In rare cases, the immune response is damaging to the body itself, resulting in anaphylaxis and other allergic reactions.

Symptoms of severe allergic reactions can include:

over-excitement
convulsions
vomiting and diarrhoea
swelling (especially of the face and throat)
shock
difficulty breathing
death

Anaphylaxis is a life-threatening medical emergency and needs immediate treatment.

Examples of Type I immune disorders:

hives and swelling (reaction to systemic, food and environmental allergens)
rhinitis (nasal inflammation)
chronic allergic bronchitis (reaction to inhaled allergens)
food allergies (reaction to certain food proteins)
skin allergies (reaction to environmental allergens)

Type II: Auto-antibody diseases (cytotoxic reactions)

The Type II immune disorders are associated with the immune system attacking the body’s own cells and causing damage and cell death. This occurs when an antibo

Blood disorders in which red blood cells and platelets are destroyed are the most common Type II reactions. Type II disorders can also affect the skin and muscles.

A common example of a Type II hypersensitivity reaction in dogs is immune-mediated haemolytic anaemia (IMHA), in which the immune system attacks the dog’s own red blood cells. The body’s organs are thus starved of oxygen, while excessive haemoglobin in the bloodstream places a heavy burden on the kidneys. There is also a high risk of blood clots forming. This is a medical emergency and requires immediate treatment.

Symptoms of this disorder can include:

jaundice (yellowing of the skin, eyes and gums)
pale gums
low appetite
low energy
weakness
low blood pressure
irregular heartbeat
dark urine

Genetically, it is most common in American cocker spaniels and field spaniels, and treatmType III: Immune complex disorders

When antibodies bind to antigens, they create a molecule called an ‘immune complex’ or an ‘antigen-antibody complex’. This molecule can get stuck in blood vessels, causing the immune system to react to the molecule and cause inflammation in the nearby tissue. The resulting disorders and diseases are called immune-mediated disorders.

Examples of Type III disorders include:

Glomerulonephritis: When the immune complex lodges in the microscopic filters (called glomeruli) in the kidneys, it causes inflammation and subsequent damage to the glomeruli. This results in excess protein in the urine and it can cause renal failure if not treated early enough.
Lupus: Also called systemic lupus erythematosus, lupus is a rare systemic disease caused by antibodies in the blood that cause an immune attack on multiple bodily organs, systems and tissues – including the blood, heart, skin, joints, lungs, kidneys, nervous system, spleen and thyroid gland. Lupus is often mistaken for another disease because of a wide variety of similar symptoms. It can be acute or chronic.
Vasculitis: When blood vessels become inflamed, it affects the surrounding tissue, creating red marks, sores and scabs on the skin in the location of the body that is affected (ears, mouth, lower legs, paws, etc.).
Immune-mediated arthritis: When immune complexes end up in or near the joints, it causes inflammation and swelling in the joints. Immune-mediated arthritis is also called canine rheumatoid arthritis, and occurs mainly in the toe, wrist (carpal) and ankle (tarsal) joints, which will feel warm to the touch. Idiopathic polyarthritis is another type of immune-mediated arthritis where the cause is unknown and it can affect any of the joints in the body, accompanied by persistent fever that is not affected by antibiotics.
Immune-mediated meningitis: Inflammation of the tissue covering the brain and spinal cord caused by invasive immune cells, with symptoms such as neck stiffness and pain, fever, and a reluctance to get up and move around. It can become a chronic condition in some pets, and large breed dogs are more susceptible to this than others.
Anterior uveitis: When the immune complex molecule affects the eyes – specifically the iris – it causes this inflammatory condition. Can occur in dogs and cats.

Type IV: Cell-mediated reactions

These types of immune disorders are caused by white blood cells reacting to specific antigens and producing toxic, cell-damaging substances that affect healthy tissues in the body. Cell-mediated reaction disorders can affect any organ in the body, the treatment for which is usually anti-inflammatories and immunosuppressants.

Old-dog encephalitis: This cell-mediated brain inflammation can occur in old dogs that had distemper earlier on in their lives.
Contact hypersensitivity: This occurs when the skin tissue is damaged by cell-mediated reactions to changes in skin proteins. Plastic food bowls, collars, and certain medications may alter the skin proteins and cause the contact hypersensitivity.
Autoimmune thyroiditis: This may be a symptom of lupus, in which the thyroid is attacked by its own immune system.
Autoimmune adrenalitis: An immune attack on the adrenal glands, leading to inflammation and the destruction of adrenal tissues.
Keratoconjunctivitis sicca: Also called ‘dry eye’, this condition is caused by the inflammation and destruction of the tear gland. The dog would either have a genetic predisposition to the condition or it would be a disordered response to a viral or bacterial infection and subsequent treatment. This condition often requires treatment with drops for the rest of their lives.

Immune-deficiency disease

Dogs and cats can also experience immune-deficiency disease, which is genetically inherited or it can be triggered by cancer or viral infections. Any disease or disorder that affects the animal’s immune system (to be immune-deficient) makes them susceptible to bacterial infection. Antibiotics do little to combat the invasive bacteria. Any wounds are also slow to heal, which perpetuates the cycle of infection.

When puppies don’t receive enough antibodies from their mothers at birth, they can acquire an immunoglobulin deficiency. In some breeds of dogs this is congenital, while it can also be acquired from disease such as cancer and viral infections like parvovirus and distemper.

Cats can be affected by immunodeficiencies when exposed to viral infections like feline panleukopenia (FPV), feline leukaemia virus (FeLV), and feline immunodeficiency virus (FIV). Fortunately, there are vaccinations for these diseases and most cats will be protected when their vaccination schedule is up to date.

Tumours of the immune system

Immune cells are required to grow and react quickly to invasive antigens. When this cell growth gets out of control, it produces cell mutations and the development of lymphomas. Lymphomas are tumours of the immune system and are the most common manifestation of cancer in older dogs. These tumours can appear in a number of places in the body and can affect numerous organs. They will usually be accompanied by swollen lymph nodes in the region of the lymphomas – if not already in the lymph nodes themselves. A dog with lymphoma will show symptoms like weakness and lethargy as well as fever.

When to see the vet about your pet’s immune system issues

Immune system problems may show up in a variety of symptoms that look like other diseases or disorders. Gastrointestinal upset may look like a symptom of indiscriminate eating or toxicity, when in fact, it could be the result of an over-active immune system.

This is why it’s so important to take your dog or cat to the vet when they appear to be unwell, instead of simply waiting to ‘see what happens’. Chronic infections, fever, itchy skin, vomiting and diarrhoea, low appetite, depression, weakness – these symptoms can be caused by a wide variety of other diseases, or they can be immune-related. The vet will run a battery of tests to determine the cause of your pet’s illness. If it’s immune-related, the treatment may involve immunosuppressant drugs to calm down an over-reactive immune response; or it may involve specific and supportive treatment for the symptoms, and therapies that do not over-burden a weak immune system.

If you have any concerns about your pet’s immune system or unexplained symptoms, always contact the vet as your first port of call.