The Innate Immune System: Second Line of Defense

Clinical Note: Some highly resilient pathogens, such as Mycobacterium tuberculosis, can actually resist digestion by the acidified enzymes within the phagolysosome, allowing them to survive inside the macrophage.

Natural Killer (NK) Cells

Unlike phagocytes, Natural Killer (NK) cells are nonphagocytic lymphocytes that patrol the blood and lymph. Their primary job is to identify and destroy "abnormal" cells, such as cancer cells or virus-infected cells, before the adaptive immune system is even activated. They do this by detecting cells that lack "self" cell-surface receptors and inducing apoptosis—a form of programmed cell death.

The Inflammatory Response: A Tissue Reaction

Inflammation is a vital tissue response to injury, whether caused by trauma, heat, irritating chemicals, or infection. While often uncomfortable, inflammation is essential because it prevents the spread of damaging agents, disposes of debris, and alerts the adaptive immune system.

The response is characterized by four cardinal signs: redness, heat, swelling, and pain. The process unfolds in three distinct stages:

1. Inflammatory Chemical Release

When tissues are injured, immune cells (like mast cells) and injured cells release chemical mediators into the extracellular fluid. Key mediators include:

• Histamine: Released by mast cells, it is a primary driver of the inflammatory response.
• Prostaglandins and Kinins: These contribute to pain and inflammation.
• Cytokines: These chemical messengers regulate the intensity and duration of the immune response.

2. Vasodilation and Increased Permeability

These chemicals trigger two major vascular changes:

• Vasodilation: Local arterioles dilate, causing hyperemia (increased blood flow) to the area. This results in the characteristic redness and heat of inflammation.
• Increased Capillary Permeability: Capillaries become "leaky," allowing exudate (fluid containing antibodies and clotting factors) to move into the tissue. This accumulation of fluid causes edema (swelling), which can press on nerve endings to trigger pain.

3. Phagocyte Mobilization

To bring "reinforcements" to the site, the body undergoes a highly coordinated mobilization process:

1. Leukocytosis: The bone marrow releases an increased number of neutrophils into the bloodstream in response to chemical signals.
2. Margination: Endothelial cells in the capillaries project cell adhesion molecules (CAMs) that grab passing neutrophils, causing them to slow down and roll along the vessel wall.
3. Diapedesis: The neutrophils flatten and squeeze through the gaps between endothelial cells to enter the interstitial space.
4. Chemotaxis: Once in the tissue, the neutrophils follow a chemical gradient (using inflammatory chemicals as "guides") to reach the exact site of injury.

Other Systemic and Chemical Defenses

Beyond cells and local inflammation, the innate system utilizes specialized proteins and systemic physiological changes to combat infection.

Antimicrobial Proteins

These proteins attack microorganisms directly or hinder their ability to reproduce:

• Interferons (IFNs): These are immune-modulating proteins secreted by virus-infected cells. They "warn" neighboring healthy cells, stimulating them to produce proteins that block viral reproduction.
• Complement Proteins: This system consists of about 20 blood proteins that circulate in an inactive form. When activated (via the classical, lectin, or alternative pathways), they trigger a cascade that enhances inflammation, promotes phagocytosis through opsonization, and can cause direct cell lysis by forming a membrane attack complex (MAC) that punches holes in the microbe's membrane.

Fever

Fever is a systemic (body-wide) response to infection. When leukocytes and macrophages encounter foreign substances, they secrete pyrogens, which act on the hypothalamus to raise the body's temperature set-point.

Exam Tip: Benefits of Fever While high fever can be dangerous, a moderate fever serves two protective purposes:

1. It causes the liver and spleen to sequester iron and zinc, which are nutrients that microorganisms need to grow.
2. It increases the metabolic rate, which speeds up the rate of tissue repair.