There are two critically important aspects of adaptive immunity. the first is specificity, while the second is

Memory

Explanation:

The two important features that characterize the adaptive immune system are specificity and memory. The ability of the adaptive immune system to target specific pathogen and generate a tailored immune response against it is termed as specificity. The ability of the adaptive immune system to remember the first exposure to the pathogen and thus, generate a heightened and more rapid immune response against the same pathogen on subsequent exposure is termed as memory. 

The  answer is 1 amu.

The answer would be semen as a whole but the cells contained are called sperm and they fertilize the eggi hope this