2. White blood cells are most closely associated with which two body systems?

(1) circulatory and digestive
(2) immune and circulatory
(3) digestive and excretory
(4) excretory and immune

(2) immune and circulatory

Explanation:

Blood is a tissue in organisms made up of cells. The blood, in addition to some organs like heart are part of the circulatory system that transports blood and nutrients throughout the human body. Blood like every other tissue is made up of cells viz: Red blood cells, White blood cells and platelets.

The white blood cells, also called leukocytes, are one of the major components of the blood. Since they constitutes what makes up the blood, they are part of the circulatory system. The white blood cells, however, have other specific functions asides being part of the circulatory system.

The white blood cells helps combat foreign invaders e.g. pathogenic microbes in the body. The white blood cells are referred to as the "SOILDERS" of the body as they help prevent infection by fighting against these disease-causing microbes. Due to this function, white blood cells play a pivotal role in the body's immunity and hence are a part of the IMMUNE SYSTEM.