Which cell type differentiates into macrophages once it leaves the blood and migrates to the tissues?

A. Calcium: Calcium reabsorption primarily occurs in the distal convoluted tubule, not the loop of Henle.

B. Drugs: Drug secretion mainly happens in the proximal convoluted tubule.

C. Urea: Urea is secreted by the loop of Henle, especially in the thin descending limb, as part of the concentration of urine.

D. Bicarbonate ions: Bicarbonate is reabsorbed mainly in the proximal convoluted tubule, not secreted by the loop of Henle.

A. Producing early induced proteins: While natural killer cells can produce cytokines, their primary function is not related to protein production.

B. Promoting opsonization of bacteria cells: Opsonization is primarily performed by antibodies and complement proteins, rather than natural killer cells.

C. Secreting signaling molecules: While natural killer cells do secrete signaling molecules (cytokines) to communicate with other immune cells, their main role is the direct destruction of infected cells.

D. Destroying virally infected cells: The primary function of natural killer cells is to recognize and eliminate virally infected cells and tumor cells, contributing to the innate immune response.