What leads to long-term immunity in mammals?

The adaptive or acquired immunity leads to long term immunity in mammals'

This arm of the immune system produces memory cells that live long.

On the first encounter with a potential pathogen, the immune system produces special lymphocyte cell that have the ability to keep memory of  the pathogen. These lymphocytes are called memory cells.

On a second encounter with the same type of pathogen, the memory cells recognize or remember the intruder and trigger off a vigorous and very specific immune response  against the pathogen, effectively neutralizing or killing it.

It is the ability of the adaptive immune system to store memory of intruders that confers mammals long term immunity.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.