This is the 12153rd most frequent Russian word.
иммунная
"Иммунная" means "immune" in English, relating to the immune system.
Here, 'иммунная' functions as an adjective describing 'система', referring to the biological system related to immunity.
Иммунная система защищает организм от болезней.
The immune system protects the body from diseases.
In this case, 'иммунная' is used to describe the type of 'реакция', which is related to immunity.
Особенности иммунной реакции зависят от состояния здоровья.
The characteristics of the immune response depend on the state of health.
Here, 'иммунная' modifies 'память', indicating a type of memory specific to the immune system.