A Cat's Immune System Explained with Cookies Title

Photo Credit: Thomas via Flickr

Staying healthy is no easy task! Your cat’s immune system works day and night to keep your cat free from disease.  If you want to help your cat keep their immune system healthy and strong, you have to have a basic knowledge about how a cat’s immune system works. The basics for cats are the same as any mammal including you and me. Things that are more specific to cats will be discussed over the next couple of weeks.

The First Line of Defense

Your cat’s immune system starts on the outside of his/her body. If a virus, bacteria, fungus or another object that is foreign to the cat’s body wants to get inside, it will have to get past the innate immune system first. The innate immune system is composed of the parts of the body’s immune system indiscriminately block out all foreign objects. The innate immune system includes the following and more:

  • the skin
  • stomach acid
  • the mucus lining the intestines
  • the mucus of the respiratory system
  • parts of the cat’s saliva
  • phagocytes (more about these below)

3 Main Players On the Inside of a Cat’s Immune System


Cookies represent potentially disease-causing foreign invaders in this analogy.

If somehow a foreign object happens to make it past the cat’s innate immune system, it’s fight isn’t over yet. They will still have to pass through the cat’s adaptive immune system. The adaptive immune system is the parts of the cat’s immune system that can “learn” about different foreign objects and develop better strategies for removing them if exposed to them again.

Let’s talk about those foreign objects for just a moment.  For the sake of analogy, let’s say that the objects that are foreign to the cat’s body are like cookies. There are all kinds of different cookies out there. All of the foreign objects that are living cells have these molecular structures on the outside of them called antigens. These antigens are like the chocolate chips, sprinkles, or nuts that might be on the cookie. Antigens come in different shapes and sizes and one foreign object could have more than one type of antigen on it. However they are arranged, the antigens help to identify the particular type of foreign body, just like chocolate chips help to identify a chocolate chip cookie.

Here are the 3 main players trying to keep “cookies” out of the cat’s body:

  1. Phagocytes
    These white blood cells are analogous to the Cookie Monster from Sesame Street. They identify foreign objects (cookies) and kill them by eating and digesting them. These cookie monsters are considered to be a part of the innate immune system because they eat the cookies no matter what kind of cookie they happen to be. However, sometimes they will present an antigen (chocolate chip) to members of the adaptive immune system to alert them to the type of invader that is present.


    Image credit for the Monocytes: Dr. Graham Beards via Wikimedia Commons

  2. T-Cells
    Going with the Sesame Street character theme, the T-cells in the cat’s immune system are like the Martians. On Sesame Street, these characters go around the planet trying to identify different earthly objects with their “Earth Book.”They have limited vocabulary like “yip, yip, yip, uh-huh,uh-huh” and “nope, nope, nope.”T-cells are “born” in the bone marrow and then they go to the thymus where they “learn” to identify a specific antigen. Each T-cell will be equipped with an antigen receptor that the antigen will fit into perfectly to form a bond. So one space alien learns identify chocolate chips while another learns to identify walnuts, etc.When a T-cell positively identifies the antigen it is trained for, it binds to that antigen (“yip, yip, yip, yip, uh-huh, uh-huh”). Then it mounts its attack. The T-cell may destroy the foreign body itself or create chemical messengers called lymphokenes that alter the phagocytes (cookie monsters) to the presence of the foreign body (cookie).


    Photo Credit for the T-cell image: NIAID via Flickr


  3. B-Cells
    B-cells are very much like T-cells and would also be martians in our Sesame Street theme. Just like the T-cells, the B-cells get their start in the bone marrow, but they either stay in the bone marrow for their “education” or are sent to the fetal liver. They also are trained to identify a specific antigen (chocolate chips, nuts, etc).An important differentiation of B-cells from T-cells is the way they attack a foreign body. Once they have bonded to an antigen, they create proteins called antibodies which can bind to the same type of antigen that the B-cell that created them binds to. Antibodies can deactivate viruses, neutralize toxins, break foreign bodies apart, or make the foreign object (cookie) more appealing to phagocytes (cookie monsters).


    B-Cell image is in Public Domain

Here is an example of each cell at work (click to enlarge):

Cookie Invading T-cell reaction

A T-Cell fighting off a dangerous fungus.

Cookie Invading B-Cell Reaction

A B-Cell fighting off the same dangerous fungus.

The Cat’s Immune System Doesn’t Fight Alone

Your cat’s immune system doesn’t fight alone either. Plenty of microbes call your cat’s body home. Don’t start wiping your cat down with disinfectants, it’s a good thing that the microbes are living on your cat! These help to train the cat’s immune system about the differences between good and bad foreign bodies. At times, these microbes even release antibiotics and other chemicals that fend off attacks from foreign bodies. It is a mutually beneficial situation for your cat and the microbes.

What things do you do to help keep your cat healthy?

Sources & Digging Deeper

Anatomy and Function in the Immune System of Cats – PetEducation.com

Phagocytes and Their Relatives – NIAID

T-Cells – NIAID

B-Cells – NIAID

Your Immune System: Natural Born Killer – Crash Course Biology

Live Dirty, Eat Clean! Why the Microbiome is the Future of Medicine – Dr. Robynne Chuktan