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.

Rescue kitten. Photo: Danielle of Central Florida Fosters.

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.