Which is the first part of the immune system to attack an invader?

Individuals do not compete directly for resources, but are prey for the same predator

Gregor mendel is best known for his work with pea plants and for uncover many of the mysteries of genetics. one of his major findings stated that there were specific, physical units of inheritance that are transmitted during reproduction. what is the the name given to these units of inheritance which can be found on chromosomes? a) centromeres b) cytoplasm c) genes d) nucleotides

This is a biological reaction due to a stimulus. i need this quickly !