Anatomy and Physiology

Phagocytes are white blood cells (leukocytes) that engulf pathogens by phagocytosis. They include neutrophils, monocytes, and eosinophils. Monocytes enlarge into large phagocytic cells called macrophages.

Natural killer cells (NK cells) are lymphocytes (white blood cells that mature in lymphoid tissues). NK cells kill pathogen‐infected body cells or abdominal body cells (such as tumors).

Complement proteins are a group of about 20 proteins that “complement” defense reactions. These proteins help attract phagocytes to foreign cells and help destroy foreign cells by promoting cell lysis (breaking open the cell).

Interferons (IFNs) are substances secreted by cells invaded by viruses that stimulate neighboring cells to produce proteins that help them defend against the viruses. Certain IFNs (such as gamma‐IFN) also amplify the activity of macrophages and NK cells.

The inflammatory response is a series of nonspecific events that occur in response to pathogens. The response typically produces redness, swelling, heat, and pain in the target area, and often the area is disabled. When skin is damaged, for example, and bacteria, other organisms, or toxic substances enter the body, the following events occur:

1. A chemical alarm is generated in the injured area. Injured cells and nearby circulating cells release chemicals that initiate defensive actions and sound an alarm to other defense mechanisms. These chemicals include histamine (mostly secreted by basophils, white blood cells found in connective tissue), kinins, prostaglandins (PGs), and complement proteins.

2. Vasodilation (dilation of blood vessels), stimulated by histamine and other chemicals, increases blood supply to the damaged area. This causes redness and an increase in local temperature. The increase in temperature stimulates white blood cells and makes the environment inhospitable to pathogens.

3. Vascular permeability increases in response to alarm chemicals. As a result, white blood cells, clotting factors, and body fluids move more quickly through blood vessel walls and into the injured area. The increase in body fluids that results causes local edema (swelling). Edema may produce pain if nearby nerve endings experience pressure. Pain may also occur when nerve endings are exposed to bacterial toxins, kinins, and prostaglandins. (Aspirin reduces pain by inhibiting the production of prostaglandins.)

4. Phagocytes arrive at the site of injury and engulf pathogens and damaged cells. Phagocytes find the site of injury by chemotaxis, the movement of cells in response to chemical gradients (provided here by alarm chemicals).

5. Complement proteins help phagocytes engulf foreign cells and stimulate basophils to release histamine.

Fever is a total body response to infection characterized by elevated body temperature. An elevated temperature increases cellular metabolism (accelerating cellular repairs), amplifies the effect of alarm chemicals, and creates a hostile environment for bacteria. An excessively high fever may reduce the activity of enzymes necessary for cellular metabolism, thus causing the body to go further out of homeostasis.