Protection from Invasion,
The Immune Response:

The DNA code protects itself.

Each species has developed an immune response to protect themselves from invasion by foreign protein that might try to destroy them, thus preventing the passage of their DNA to the next generation.

The immune response developed at the level of the cell itself. 

The dilemma is.....

A. How can a member of the species obtain the amino acids that it needs without becoming the victim of a strange protein?

B. How can the cells of one individual "fertilize" the cell of another individual without being rejected as a foreign protein?

In problem "A", the immune response is
by-passed by the process of digestion.

Each organism preserves the integrity of its species while providing food for its cells. Your cells then convert the amino acids back into your own proteins.

In problem "B", the immune response is 'neutralized'.

In mating, male cells carry an agent whose job is to defeat the "immune response" of the target female cell.

Once the immune response is neutralized the sperm cell easily invades the egg cell where it can transfer its DNA into the nucleous.