What is a major cause of an increase in the number of leukocytes in the blood?

An increase in the number of leukocytes, or white blood cells, is primarily associated with the body's immune response to various conditions, particularly infections. When pathogens such as bacteria, viruses, or fungi invade the body, the immune system reacts by producing more leukocytes to help combat these threats. This elevated production serves to enhance the body's ability to seek out and eliminate the invading microorganisms. Therefore, when an infection is present, the leukocyte count rises significantly as part of the overall defense mechanism.

Other options, while they may influence blood cell counts under certain circumstances, do not primarily provoke the same direct response in leukocyte levels associated with infections. Anemia typically involves a decrease in red blood cells rather than an increase in leukocytes. Dehydration can concentrate blood components, potentially raising white blood cell counts but not as a result of increased production in response to pathogens. Allergies can cause an increase in specific types of leukocytes, such as eosinophils, but this is a more nuanced reaction involving different immune mechanisms compared to the broader leukocyte response triggered directly by infections.