Here's why:

* White blood cells (leukocytes): These cells are responsible for fighting infections and contain a nucleus, which houses the DNA.

* Red blood cells (erythrocytes): While red blood cells are the most numerous in blood, they lack a nucleus in their mature form. This means they don't contain DNA.

* Platelets: These small cell fragments involved in clotting also lack a nucleus and therefore don't contain DNA.

Therefore, when we extract DNA from blood, we are primarily extracting it from the white blood cells present in the sample.