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.