I’m referring to the domestic cat. A good source on the Researchgate website, a study entitled Immune System Development in the Dog and Cat by M. J. Day published August 2007 on the Journal of Comparative Pathology, tells me that, “immunocompetentcy occurs around 1.5-3.0 months of age”. I’m requesting the full text of this study because it may be the case that cats are considered to be fully immunocompetent from the age of twelve months. This is because Google finds the following quote from the M.J. Day’s research:
Cats are considered fully immunocompetent from the age of 12 months. There are still substantial changes in the immune system during the first year of life (Day, 2007)
I’ll be able to confirm that statement when I receive the full text of the research. Currently I can only access a bit of it which does not provide me with a clear answer. The age at which kittens look like adults is around 12 months.
I confess that I have searched high and low for a definitive answer as to when a cat’s immune system is fully developed without being confident of the outcome of my research. One fact is clear, however, which is that a mother’s antibodies provided in her milk (colostrum) provide passive immunity to her offspring and they disappear at between six and sixteen weeks of age (Cat Owner’s Home Veterinary Handbook). Logically you would have thought that a kitten’s immunity would therefore be developed at around sixteen weeks of age which is about four months.
Kittens younger than three weeks old may be incapable of developing antibodies in response to a vaccine. This may be due to either physical immaturity or interference by passive maternally acquired antibodies i.e. the antibodies of the kitten’s mother. These antibodies can bind the antigen in the vaccine which prevents it from stimulating an immune response in the kitten being vaccinated.
Watch this space while I try and find a definitive answer. It is remarkable, actually, that it is almost impossible to find a clear answer to the question in the title on the internet. Neither do my books have an answer on this. The reason may be because the experts don’t know when a domestic cat’s immune system is fully developed. And the time frame may well vary between individual cats.
Update September 14, 2022: Note: there is an overlap with what is written below and above as I search for useful information. The purpose of this update is to try and add a more precise timetable to when a cat’s immune system kicks in plus some other useful information I hope. I’m talking about an ‘endogenous immune response’ that is an immune response which is not provided by the antibodies of the mother’s colostrum, or the antibodies provided by the mother when the kitten was an embryo. The kitten initially relies upon the mother and then at a certain time they produce their own immune system.
The Auburn Animal Hospital in Chicago, Illinois, states that a kitten’s immune system is fully developed at about 12 months of age.
If the mother is healthy kittens receive about 25% of their needed serum antibodies for temporary immunity via the placenta before birth. During the first 36 hours after birth mothers produce colostrum which is high in vitamins, minerals and protein. It contains antibodies and other immune substances primarily IgG which provides protection against infectious diseases.
If kittens don’t nurse from a cat during the first 24 hours owners should consider giving them serum from any healthy well vaccinated adult cat. The serum should be injected under the skin at about 1 mL per pound of kitten bodyweight. This will provide immunity for about six weeks.
Kittens are best able to absorb antibodies from their mother’s milk during the first 24-36 hours of life.
As long as the antibodies are inside the kitten’s circulation, they have an immune reaction. Mother cats that have been vaccinated just before they were bred have the highest antibody levels. They can protect kittens for up to 16 weeks.
There are difficulties in vaccinating young kittens. Kittens younger than three weeks of age may not be able to develop antibodies in response to a vaccination. It is believed that the mother’s antibodies combine with the antigen in the vaccine preventing it from stimulating the immune system, but these “passive antibodies” disappear at about 6-16 weeks of age.
There is an “immunity gap“. This is the time between the antibodies provided by the mother no longer protect the kitten, but they are too high inside the kitten to allow vaccines to be effective. The kitten is therefore more vulnerable to disease. Ensuring that kittens have a warm, comfortable and quiet place to sleep helps to strengthen their immune system.
Below are some more articles on newborn kittens.