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Explores the body's non-specific first and second lines of defense against pathogens.
Every second, your body is under siege by millions of invisible invaders. How does your system distinguish between a harmless dust mote and a deadly pathogen without ever having met either one before?
The innate immune system is your body's non-specific security force. It doesn't care who the invader is; it simply stops anything 'non-self.' The first line of defense consists of physical and chemical barriers. The skin is a formidable physical wall, reinforced by a keratinized layer that is nearly impenetrable to most microbes. Chemically, the skin maintains a low (between and ) due to sebum and sweat, which inhibits bacterial growth. Mucous membranes line body cavities open to the exterior, trapping pathogens in sticky mucus. Furthermore, body fluids like tears and saliva contain lysozyme, an enzyme that physically ruptures bacterial cell walls. If a pathogen breaches these walls, it triggers the second line of defense.
Quick Check
Why is the skin's low pH considered a chemical barrier rather than a physical one?
Answer
Because it uses a chemical property (acidity) to inhibit bacterial growth, rather than providing a structural blockage.
Once a pathogen enters the tissue, cellular defenses activate. Phagocytes, such as neutrophils and macrophages, act as the body's 'clean-up crew.' They identify pathogens, engulf them via phagocytosis, and digest them using lysosomal enzymes. While phagocytes eat the enemy, Natural Killer (NK) cells act as specialized assassins. Unlike phagocytes, NK cells do not ingest pathogens. Instead, they patrol the blood and lymph to find 'altered' self-cells (like virus-infected or cancerous cells). When they find a target, they release perforins and granzymes, which punch holes in the target's membrane and trigger apoptosis (programmed cell death).
The process of a macrophage destroying a bacterium follows these steps: 1. Chemotaxis: The macrophage follows a chemical 'scent' left by the pathogen. 2. Adherence: The macrophage's receptors bind to the bacterium's surface. 3. Ingestion: The macrophage wraps its membrane around the bacterium, forming a vesicle called a phagosome. 4. Digestion: The phagosome fuses with a lysosome to create a phagolysosome, where enzymes destroy the invader.
Quick Check
How do Natural Killer (NK) cells differ from phagocytes in their method of destruction?
Answer
NK cells induce apoptosis (cell suicide) via chemical signaling, whereas phagocytes physically ingest and digest the pathogen.
The inflammatory response is a localized, non-specific heat-up designed to prevent the spread of agents to nearby tissues. It begins when damaged cells release chemicals like histamine and prostaglandins. These chemicals cause vasodilation (widening of blood vessels), which increases blood flow to the area—this is why an injury looks red and feels hot. The increased permeability of local capillaries allows fluid and clotting factors to leak into the tissue, causing edema (swelling). This swelling is actually beneficial: it dilutes harmful substances and brings in the oxygen and nutrients needed for repair. Finally, phagocytes are drawn to the area via chemotaxis to finish the job.
Imagine a wooden splinter piercing your thumb. 1. Chemical Alarm: Damaged tissue cells and mast cells release histamine. 2. Vasodilation: Blood vessels dilate, causing the thumb to turn red and feel warm (). 3. Permeability: Capillaries leak fluid, causing the thumb to swell and press against nerve endings (pain). 4. Phagocyte Mobilization: Neutrophils squeeze through capillary walls (diapedesis) to attack bacteria introduced by the splinter.
In severe infections, the innate system may trigger a fever, a systemic response. Pyrogens are released into the blood and reset the body's thermostat in the hypothalamus. If a normal body temp is , a fever might raise it to . While uncomfortable, this increase speeds up metabolic repair rates and causes the liver to sequester iron and zinc, which bacteria need to multiply.
Which enzyme found in tears and saliva is responsible for breaking down bacterial cell walls?
What is the primary cause of the 'swelling' (edema) observed during inflammation?
Natural Killer (NK) cells are considered phagocytes because they engulf virus-infected cells.
Review Tomorrow
In 24 hours, try to list the four cardinal signs of inflammation and explain the physiological cause behind each one without looking at your notes.
Practice Activity
Create a flowchart showing the journey of a bacterium from the moment it touches the skin to the moment it is destroyed by a macrophage.