One of the important agents of the immune system is

A) serotonin.
B) the heart.
C) the adrenal gland.
D) the macrophage.

Immune System

The complex network of cells, tissues, and organs in the body that works together to defend against infection, disease, and foreign substances.

Macrophage

A type of white blood cell that engulfs and digests cellular debris, foreign substances, microbes, and cancer cells.

Serotonin

A neurotransmitter in the brain involved in regulating mood, appetite, and sleep, among other functions. Low levels are associated with depression.