🔥 Inflammation: How the Body Responds to Injury and Infection
🛡️ What Is Inflammation?
Inflammation is a protective response of vascularized tissues to infection, tissue injury, or cellular damage. Its major goals are to eliminate the cause of injury, remove damaged or dead tissue, and begin repair. Acute inflammation involves coordinated activity among immune cells, chemical mediators, blood vessels, and plasma proteins.
🦠 What Triggers Inflammation?
Common inflammatory stimuli include microbes and necrotic tissue. Resident immune cells recognize signals associated with pathogens or damaged cells. This recognition activates the inflammatory response and initiates the production of chemical mediators that recruit additional components of the immune system.
🔎 Sentinel Cells Recognize the Threat
Macrophages, dendritic cells, and mast cells function as important sentinel cells within tissues. When these cells detect infection or tissue damage, they release inflammatory mediators. Macrophages and dendritic cells are especially important for recognizing microbes, while mast cells can rapidly release mediators such as histamine.
🧪 Inflammatory Mediators
Inflammatory cells produce signaling molecules including amines and cytokines. Histamine promotes vascular changes, while cytokines and chemokines help activate and recruit leukocytes. These signals coordinate the local inflammatory response and direct immune cells toward the affected tissue.
🩸 Vasodilation and Increased Permeability
Acute inflammation produces vasodilation, increasing local blood flow. Vascular permeability also increases, allowing fluid and plasma proteins to leave the circulation and enter affected tissues. The accumulation of excess interstitial fluid contributes to edema, or swelling.
🧬 Recruitment of Leukocytes
Inflammatory mediators recruit circulating leukocytes to the site of injury. Neutrophils generally dominate early in many acute inflammatory responses, while monocytes subsequently enter tissues and differentiate into macrophages. Leukocyte recruitment involves interactions with vascular endothelium followed by migration into the tissue.
| 🧬 Component | 🔥 Major Role in Inflammation |
|---|---|
| Macrophage | Recognition, phagocytosis, and cytokine production |
| Dendritic cell | Antigen recognition and presentation |
| Mast cell | Rapid mediator release, including histamine |
| Neutrophil | Early phagocytosis and microbial killing |
| Monocyte | Enters tissue and can differentiate into a macrophage |
| Fibroblast | Produces extracellular matrix during tissue repair |
⚔️ Elimination of Microbes and Dead Tissue
Once leukocytes reach the affected area, they help eliminate microbes and remove damaged cellular material. Phagocytosis allows neutrophils and macrophages to engulf pathogens and debris. Intracellular antimicrobial mechanisms then help destroy engulfed microorganisms.
🧱 Resolution and Tissue Repair
After the inflammatory stimulus has been controlled, the response should move toward resolution and repair. Macrophages release cytokines and growth factors that influence fibroblasts and other cells. Fibroblasts produce components of the extracellular matrix, helping reconstruct damaged tissue. Depending on the injury, healing may involve regeneration, scar formation, or both.
🧠 MCAT High-Yield Takeaway
For the MCAT, remember the general sequence: injury or microbes → recognition by sentinel cells → inflammatory mediator release → vascular changes → leukocyte recruitment → elimination → repair. Also connect vasodilation with increased blood flow, increased vascular permeability with edema, neutrophils with early acute inflammation, and macrophages/fibroblasts with cleanup and repair.