Under the Weather: A Look Into the Feline Immune System

The immune system is a complex network of specialized cells and organs designed to defend the body against bacteria, viruses, toxins, parasites and any foreign material that invade the cat’s body. Millions of different types of immune cells pass information back and forth, resulting in a extremely protective system that is always ready to produce a fast and effective immune response. It is responsible for recognizing and fighting off foreign substances called antigens.

Interestingly, the immune system has the ability to distinguish between the “self” cells (cells of its own body) or “nonself” substances (foreign substances). Every cell in the body carries a molecule that identifies it as “self”, so that the immune system does not attack its own tissues.

Types of immune Responses

The immune system can be divided into two parts based on how specific their functions are. These two divisions are called the innate immune system and the adaptive immune system. The first system is the first line of defense. If the invader is stopped by the innate system, no disease will occur and the body will continue working properly. However, if the invader cannot be stopped by the innate system, the adaptive system is activated so that the body could recover. If neither innate or adaptive systems are effective, death can occur.

All organisms have what could be considered an innate defense system. For trees, it would be their bark. For mammals and reptiles, it would be their skin, and for a bacterium, it would be the cellular wall around it.


This is often the first line of a defense. It is nonspecific, meaning it is designed to keep everything out. It is also characterized as non-adaptive, meaning that its effectiveness is not changed by repeated exposure to a foreign substance. In addition to the skin, stomach acid, and mucus of the respiratory system, special chemicals in the saliva are all part of this innate system. They also have certain cells in the body called phagocytes, monocytes, and macrophages. These cells will basically eat anything foreign that is in sight.

In addition to the innate system, organisms also have an adaptive immune system. This part of the immune system defends the body against specific foreign invaders. After successfully fighting off the foreign bodies, they retain memory of past invaders. Parts of the adaptive system also communicate with each other and share information of the various invaders encountered. So if a second exposure occurs, the adaptive system will mount a greater and faster response, becoming much more effective.

Major Components

Organs of the immune system are often called lymphoid organs because they are frequently the site of growth, development, and deployment of lymphocytes – white blood cells that are key operatives in the immune system. They are located throughout the body.

Immune cells are typically concentrated in the blood, thymus, lymph nodes, bone marrow, spleen, lungs, liver, and intestines. These organs are all connected with one another and with other organs of the body by a network of lymphatic vessels, similar to blood vessels. Immune cells, proteins, and sometimes foreign particles are carried through these vessels in a clear fluid that bathes the body’s tissues. The various components are also linked by the circulatory system.

Here are some of the major organs of the immune system:

Lymph nodes – These are small, bean-shaped structures lying across the course of lymphatic vessels in sites such as the neck, armpit, and groin. They filter and trap antigens, the portion of a virus or bacteria that caused an immune response, from the lymphatic vessels and the blood stream.

Spleen – This organ is located in the upper left quadrant of the abdomen. Its function is to filter and trap antigens directly from the blood stream.

Bone marrow – The marrow cavity is located in the center of several bones in the body. The bone marrow is the site of production of many white blood cells.

Thymus – The thymus is located in the front part of the chest, just in front of the heart. It is the largest in the young animal when the development of the immune system is at its most active. As the animal matures, it shrinks in size.

Important Cells and Proteins

There are two types of lymphocytic cells that are critical in the immune system: T-cells and B-cells. Each lymphocyte, whether a B-cell or a T-cell, is educated to identify one particular antigen. The educated cells use antigen receptors on their surface to recognize antigens, as they fit together like a lock and key.

T-cells are initially processed by the thymus gland and are responsible for cellular immunity. When receptors on a T-cell bind to an antigen, it activates the T-cell to either destroy it, or make substances called lymphokines. These are chemical messengers to the macrophages and other phagocytes, calling them “to come in and eat”.

In mammals, B-cells arise in the bone marrow. They are responsible for making antibodies that are proteins used to fight infections and foreign material. B-cells are also associated with the building of specific immunity and the specific response to produce antibodies.


Antibodies are specialized serum proteins that are produced in response to antigens. They are also called immunoglobulins, which the body produces several classes or types of antibodies depending on what the foreign body is.

Each B-cell is programmed to make one specific antibody. When a B-cell encounters its triggering antigen, it stimulates many large plasma cells, factories for producing that one specific antibody.

White blood cells, also known as leukocytes, have a wide variety  that each have a specific and special function in the immune system. Some are designed to react primarily to bacteria and inflammation, others react more to parasites and foreign material, while others assist the lymphocytes in producing antibodies.

For example, monocytes and macrophages are specific types of white blood cells that digest and kill foreign invaders. They can also serve as a part of the adaptive system by presenting portions of the antigens to other cells in the adaptive system, alerting them to the presence of the invader.


Building Immunity

Whether the body’s response is primarily humoral (through antibodies) or cell-mediated, certain T and B cells become memory cells. These cells remember their exposure to specific antigens and help the body respond much faster and with a larger response if exposed to these foreign substances again.

This is the mechanism by which vaccination helps protect the body from disease. If a cat receives a combination vaccine containing panleukopenia, rhinotracheitis, and calicivirus, three different groups of memory cells will be produced, one for each antigen. When the cat is vaccinated again three to four weeks later, the body’s response to the second vaccination will be greater and much faster than after the first vaccination. This faster and higher response is scientifically termed as an ‘anamnestic response’.

However, memory cells do not live forever. Eventually, the body will need to regenerate these cells. In some cases, animals need to be revaccination to produce a new generation of memory cells. For some diseases, this might be every year, for others three years or longer. When discussing about the duration of immunity, the length of time an animal is protected, it really about how long a sufficient number of memory cells can live and how long the antibodies will remain so that the animal is still protected.

There are two main ways in which an animal can acquire immunity.

Active immunity occurs when people or animals are exposed to a disease-causing organism by natural means or vaccination so that the antigens can interact with the cells of the animal’s immune system. It is self-perpetuating, with  more antibodies being produced whenever the immune system comes in contact with the same organism again.

innate_adaptive_systemsPassive immunity occurs when an animal receives another animal’s defense mechanisms rather than developing its own defense system. Examples include the antibodies received by a fetus through the placenta. However, a disadvantage is that the animal’s body does not have the ability to replenish its immunity, unlike active immunity.

Disorders and Diseases of the Immune System

  • Congenital immune deficiencies: It is a deficiency due to the abnormal function of one or more white blood cells, the inability to produce normal numbers of white blood cells, or an inability to produce antibodies
  • Congenital underdevelopment of the thymus
  • Acquired immune deficiencies may develop in association with other systemic diseases such as: sugar diabetes, feline leukemia virus infection, feline immunodeficiency virus, feline infectious peritonitis virus, and cancer
  • Immune-mediated diseases: any disorder in which the immune reaction mounted by the body is harmful to the body, or when the immune reaction is mistakenly directed against parts of the body’s own organs. Examples include allergic reactions, anaphylaxis,(a life-threatening allergic immune reaction), atopy or allergic skin disease from inhaled allergens; feline asthma, immune-mediated hemolytic anemia, where the body attacks its own red blood cells.
  • Cancer: usually involves the overproduction in immune cells, and may result in the overproduction of immunoglobulins.
  • Parvovirus: causes a severe and short-term reduction in the number of neutrophils and in the lymphocyte responsiveness. These failures of the immune system increase the risk of fungal infection such as aspergillosis, mucormycosis, and candidiasis.
  • Feline leukemia virus: This viral infection causes a response similar to that seen in people infected with human immunodeficiency virus (HIV). With an impaired immune system, infected cats have a higher risk of acquiring infections from bacteria and other infectious agents in the environment. This spreads from cat to cat, primarily by biting. It is a lifelong disease, although the cat may appear healthy with no further signs of infections for months or years.

References + For More Reading

Structure and Function of the Immune System in Cats

Anatomy and Function of the Immune System In Cats

The Immune System of Cats

Immune-deficiency Diseases in Cats

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Previous Feline Posts

Take A Bite: A Look Into the Feline Digestive System

Take A Breath: A Look Into the Respiratory System

Don’t Skip A Beat: A Feline’s Circulatory System

Getting On A Cat’s Nerves: A Look Into the Feline Nervous System


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