Final answer:
An increase in leukocytes, or leukocytosis, can be due to responses to infection, inflammation from injury such as burns, or chronic diseases including leukemia. This response is critical for the body's immune system to effectively fight off pathogens and facilitate healing.
Step-by-step explanation:
If blood is found to contain twice as many leukocytes as normal, the most likely cause is a condition known as leukocytosis. This condition can be an expected finding in various contexts, such as in a patient who has suffered a severe burn, as leukocytes respond to chemical signals from injured cells and pathogens.
Leukocytosis is associated with infections (viral or fungal), chronic diseases, and conditions such as leukemia. It occurs because leukocytes are part of the body's immune response; they increase in number to help the body fight off these conditions. Also, in conditions like leukopenia, where there are too few leukocytes produced, the individual can't ward off diseases effectively, demonstrating the essential role of leukocytes in the immune system.