The immune system is the body’s defense mechanism that protects against infections caused by bacteria, viruses, fungi, and parasites. It identifies harmful invaders, attacks them, and removes them from the body. The immune system works through a complex network of cells, tissues, and organs that work together to fight infections.
The immune system defends the body against infections through three main lines of defense.
The first line of defense consists of physical and chemical barriers like the skin, mucous membranes, and stomach acid, which prevent harmful invaders from entering the body.
If a pathogen bypasses these barriers, the second line of defense, the innate immune response, takes action. This includes white blood cells like phagocytes that engulf and destroy invaders, natural killer cells that attack infected cells, and the inflammatory response, which brings more immune cells to the infection site. The body may also develop a fever, which makes it harder for pathogens to survive.
If the infection continues, the third line of defense, or adaptive immune response, steps in. Here, B cells produce specific antibodies to target the pathogen, while T cells either help activate other immune cells or directly destroy infected cells.
Once the infection is overcome, memory cells stay in the body to recognize the pathogen if it returns, allowing the immune system to respond more quickly next time.
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