When pathogens attempt to enter the body, the first defense to act is innate immunity. Innate immunity is a nonspecific defense mechanism present from birth and consists of three stages: barriers formed by the skin and mucous membranes, phagocytosis, and inflammatory responses. It provides rapid and broad defense without remembering specific pathogens.
Basic Knowledge
Physical and chemical barriers: The skin consists of a keratinized layer and is the first line of defense against the physical entry of pathogens. Mucous membranes (the respiratory tract, digestive tract, and urogenital tract) eliminate foreign substances through mucus and ciliary movement. Saliva, tears, and stomach acid contain antimicrobial substances such as lysozyme.
Phagocytosis: Pathogens that break through the barriers are engulfed by phagocytes such as macrophages, neutrophils, and dendritic cells, and are broken down by enzymes in lysosomes. Macrophages secrete cytokines and initiate inflammation.
Inflammatory response: When tissue is damaged, mast cells release histamine, causing local blood vessels to dilate and become more permeable. Neutrophils and macrophages migrate from the blood into the tissue and eliminate pathogens. The four signs of inflammation (redness, heat, swelling, and pain) are manifestations of this process.
Recognition mechanism of innate immunity: Pattern-recognition receptors (PRRs), especially Toll-like receptors (TLRs), recognize common structures of pathogens (PAMPs: pathogen-associated molecular patterns). LPS (a cell-wall component of Gram-negative bacteria), peptidoglycan, and viral RNA are representative examples of PAMPs.
What does the “nonspecificity” of innate immunity mean? Explain it in contrast with adaptive immunity.
Answer: The nonspecificity of innate immunity means that it responds broadly by recognizing PAMPs (pathogen-associated molecular patterns), structures common to pathogens, rather than identifying a specific pathogen. Adaptive immunity precisely responds to and remembers specific antigens, whereas innate immunity can respond immediately (within several hours) even to a pathogen encountered for the first time.
- First line of defense = barriers such as the skin, mucous membranes, and lysozyme
- Second line of defense = phagocytosis by macrophages, neutrophils, and dendritic cells
- Four signs of inflammation = redness, heat, swelling, and pain
- Histamine = released from mast cells; vasodilation and increased permeability
- TLRs = pattern-recognition receptors that recognize PAMPs
- Dendritic cells = bridge between innate and adaptive immunity
- Innate immunity is non-memory-based, rapid, and nonspecific
Points to Note
① The greatest difference from adaptive immunity is that innate immunity does not form “memory.” ② Dendritic cells act as a bridge by presenting antigens to T cells while carrying out phagocytosis, thereby activating adaptive immunity. ③ Inflammation is not a pathological reaction but a physiological response essential for tissue repair.
Practice
- Give three examples of physical barriers in innate immunity.
- Name three representative cells that carry out phagocytosis.
- List the four signs of inflammation and explain the biological reaction that causes each one.