Q.(a) A person inhales dust and pathogens but does not always fall sick. Explain how the body protects against such infections.

The body has a multi-layered defence system that prevents inhaled dust and pathogens from causing illness. The first line of defence is physical and chemical barriers that block entry. If pathogens get past these, the innate immune system responds quickly with non-specific mechanisms. Only if that fails does the slower, specific adaptive immune system activate.

1. When you inhale dust and pathogens, the respiratory tract is lined with mucus that traps particles. Tiny hair-like cilia sweep the mucus upward toward the throat, where it is swallowed or coughed out. This mechanical clearance prevents most microbes from reaching the lungs. Additionally, enzymes in mucus and secretions can destroy pathogens directly.
2. Two important barriers involved are: …

When you inhale, the air entering your respiratory tract carries not just oxygen but also a host of unwanted particles: dust, pollen, bacteria, viruses, and fungal spores. Yet most of the time, you remain perfectly healthy. This is not luck — it is the result of a carefully layered defence system that the body has evolved. The immune system does not rely on a single wall; it builds a fortress with many gates, moats, and sentries.

The first line of defence is non-specific — it works against all invaders, regardless of their identity. Think of it as a general security system. The second line is specific — it targets particular pathogens and remembers them for future encounters. Together, these two lines ensure that most inhaled threats are neutralised long before they can cause harm.

(a) How the body protects against inhaled dust and pathogens

The moment air enters your nostrils, protection begins. Hairs in the nasal cavity trap larger dust particles. The entire respiratory tract, from the nose down to the bronchi, is lined with a moist mucous membrane that secretes mucus — a sticky, slimy fluid. Dust and microbes get trapped in this mucus like flies on flypaper.

Beneath the mucus layer lies a carpet of cilia — tiny hair-like projections that beat in a coordinated, wave-like motion. They constantly sweep the mucus (and everything stuck in it) upward toward the throat. From there, you either swallow it (stomach acid destroys most pathogens) or cough it out. This mechanism is called the mucociliary escalator.

If a pathogen manages to slip past these physical barriers and enter the lungs, it meets alveolar macrophages — large scavenger cells that patrol the air sacs. These cells engulf and digest foreign particles and microbes through a process called phagocytosis. They are the cleanup crew of the respiratory system.

On top of all this, the body’s chemical defences are always active. Mucus contains enzymes like lysozyme that break down bacterial cell walls. Stomach acid kills swallowed microbes. And if an infection does begin, the inflammatory response rushes blood and immune cells to the site, raising the local temperature and making it harder for pathogens to multiply.

(b) Two barriers involved in this protection

The NCERT classifies barriers into physical, physiological, cellular, and humoral categories. Here are two important ones:

1. Physical barrier: Skin and mucous membranes …