The Immune System and Defense

Organisms are constantly exposed to pathogens—bacteria, viruses, fungi, and parasites that can cause disease. The immune system is the body's defense network, combining physical barriers with cellular and molecular responses that detect and destroy invaders while sparing the body's own tissues.

First line: barriers

Before any specific response, the body relies on barriers that block entry: the skin, mucus, stomach acid, enzymes in tears and saliva, and beneficial microbes that outcompete pathogens. These non-specific defenses stop most threats before they take hold.

Innate immunity

If a pathogen breaches the barriers, the innate immune system responds quickly and non-specifically—the same way against any invader, with no memory of past exposure. Its features include:

• Phagocytes (such as m