Innate Immunity: The Body's First Line of Defense

Internal Defense Mechanisms

1. Antimicrobial Proteins

The body utilizes specialized proteins to signal danger or directly attack pathogens:

• Interferons (IFN): These are released by activated lymphocytes, macrophages, or cells that have already been infected by a virus. They serve as a warning signal to neighboring cells to bolster their defenses.
• The Complement System: This group of proteins works to facilitate the lysis (bursting) of pathogens, helping to destroy them directly.

2. Cell Destruction

When proteins alone are not enough, specialized cells move in to eliminate the threat through two primary methods:

• Phagocytosis: This is the process of "cell eating." Phagocytic cells, such as macrophages and neutrophils, identify, ingest, and digest microbes.
• Cell Lysis: Natural killer (NK) cells target abnormal or infected cells and induce lysis, causing the target cell to rupture and die.

3. Inflammation

Inflammation is a localized response to tissue injury. Its primary goals are to limit the spread of infection, destroy the infectious agent, and aid in repairing damaged tissue.

The process involves two key steps:

1. Vasodilation: The widening of blood vessels to increase blood flow to the injured area.
2. Chemotaxis: A chemical signaling process that attracts phagocytes to the specific site of infection.

4. Fever

A fever is an increase in body temperature above the normal $37 \text{ ºC}$. While often viewed as a symptom of illness, fever is actually a functional defense mechanism that:

• Mobilizes the body's defenses.
• Accelerates metabolic reactions.
• Enhances repair processes.
• Inhibits the growth of certain microorganisms.

💡 Study Tip: Innate vs. Adaptive

To distinguish these on an exam, remember: Innate is "non-specific" and "immediate" (the first responders), whereas Adaptive is "specific" and "acquired" (the specialized task force that includes B cells and T cells).

Summary of Immune Responses

To understand how innate immunity fits into the larger picture, it is helpful to see how it interacts with the adaptive system. While innate immunity uses "nonspecific killers" like macrophages and NK cells to mount a physical attack, the adaptive system uses antibodies (humoral immunity) and T cells (cellular immunity) to mount a more targeted chemical and cellular attack.