Understanding Fever and Inflammation as Immune Responses

1. Limit the spread of pathogens.
2. Destroy the invading pathogens.
3. Remove debris from the damaged tissue.
4. Initiate the repair process.

In medical terminology, the suffix -itis is used to denote the inflammation of a specific organ or tissue, such as arthritis (inflammation of the joints) or dermatitis (inflammation of the skin).

The Cardinal Signs of Inflammation

Inflammation is characterized by four distinct "cardinal signs" that you can observe at the site of injury:

1. Heat: This is caused by hyperemia, which is an increase in blood flow to the injured area.
2. Redness: Also a result of hyperemia and the presence of extravasated (leaked) red blood cells within the tissue.
3. Swelling (Edema): This occurs due to increased fluid filtration from the capillaries into the surrounding tissue space.
4. Pain: Pain can be triggered by several mechanisms: direct injury to the nerves, physical pressure on nerves caused by edema, or the stimulation of pain receptors by chemical mediators like prostaglandins, bacterial toxins, and bradykinin.

Regulation via Cytokines

The inflammatory process is tightly regulated by cytokines. These are small proteins secreted primarily by leukocytes (white blood cells) that alter the physiology of the cells that receive them. Cytokines can act in two ways:

• Paracrines: They affect neighboring cells in the immediate vicinity.
• Autocrines: They affect the same cell that secreted them.

Key types of cytokines involved in these processes include interferons, interleukins, tumor necrosis factor, and chemotactic factors.

Leukocyte Behavior: The Journey to the Injury Site

To effectively fight an infection, leukocytes must leave the bloodstream and travel to the exact site of the injury. This is not a random process; it involves a highly coordinated sequence of behaviors:

1. Margination

Normally, blood flows rapidly through the center of blood vessels. During inflammation, certain adhesion molecules called selectins cause leukocytes to slow down and adhere to the inner walls of the blood vessels. This process of moving toward the vessel wall is known as margination.

2. Diapedesis (Emigration)

Once adhered to the wall, the leukocytes must exit the circulatory system. Through a process called diapedesis (or emigration), the leukocytes squeeze between the endothelial cells that line the blood vessel to enter the interstitial tissue space.

3. Chemotaxis

Once in the tissue, the leukocytes do not wander aimlessly. They are guided to the specific site of injury by a chemical "scent trail" through a process called chemotaxis. Chemicals such as bradykinin and leukotrienes act as attractants, guiding the leukocytes directly to the pathogens or damaged cells.

Summary Table: Leukocyte Movement

By coordinating these responses—the systemic rise of fever and the localized intensity of inflammation—the immune system creates a multi-layered defense capable of neutralizing threats and initiating the complex work of healing.