Body Defences: The First and Second Lines

1Why this matters

Every minute you breathe in, swallow and touch millions of microorganisms, yet you are healthy most of the time. That is because your body defends itself in layers, like a school compound: a wall and gate that keep most intruders out, guards who deal with anyone who climbs in, and specialists who are called when the guards cannot cope.

Biologists divide these defences into two systems. The innate (non-specific) system is present from birth, acts within minutes and treats every invader the same way. It is made up of the first line of defence (barriers) and the second line of defence (internal responses). The adaptive (specific) system is the third line, which you will study in the next lesson.

2The first line of defence: keep pathogens out

• Skin. The outer layer is made of dead, flattened cells packed with a tough protein called keratin. Unbroken skin is almost impossible for microorganisms to cross. Sweat and oil make its surface slightly acidic, which slows the growth of bacteria.
• Mucous membranes and mucus. The linings of the nose, airways, gut and reproductive tract produce sticky mucus that traps microorganisms and dust.
• Hairs and cilia. Hairs in the nose filter large particles. The trachea and bronchi are lined with cilia, tiny hair-like projections that beat together and sweep mucus, with its trapped microbes, up to the throat, where it is swallowed or coughed out. Cigarette smoke paralyses cilia, which is one reason smokers suffer more chest infections.
• Chemical defences. Tears, saliva and mucus contain lysozyme, an enzyme that breaks down the cell walls of bacteria. The stomach makes hydrochloric acid (about pH 2), which kills most microorganisms swallowed with food.
• Harmless resident microorganisms. Bacteria that normally live on the skin and in the gut use up the space and food that harmful microbes would need.
• Blood clotting. When the skin is cut, platelets and blood proteins quickly form a clot that seals the gap.

3The second line of defence: fight whatever gets in

Pathogens do get past the barriers, through a cut, a mosquito bite, or in numbers too large for mucus to hold. The second line then responds, in the same way whatever the invader is.

• Inflammation. Damaged or infected tissue releases chemicals such as histamine (from cells called mast cells). Nearby capillaries widen, so more blood flows in, causing redness and heat. Their walls also become leaky, so fluid escapes into the tissue, causing swelling, and the swelling presses on nerve endings, causing pain. The leaky capillaries let white blood cells and defensive proteins reach the infected site quickly.
• Phagocytosis. Phagocytes are white blood cells that engulf and digest pathogens. Neutrophils are the most numerous and arrive first; macrophages are larger, arrive later and clean up dead cells and debris. Pus is a mixture of dead neutrophils, dead bacteria and tissue fluid.
• Fever. Chemical messengers released by white blood cells raise body temperature. A moderate fever slows the growth of many pathogens and speeds up repair. A very high fever, above about 40 °C, is dangerous and needs medical care.
• Interferons. Cells infected by a virus release these proteins as a warning. Neighbouring cells respond by making substances that stop the virus from reproducing inside them.
• Natural killer cells. These white blood cells recognise body cells that have been infected by viruses, or that have become cancerous, and make them destroy themselves.
• Complement. A group of about 20 proteins in the blood. They coat pathogens, making them easier for phagocytes to engulf, and some punch holes in bacterial membranes.

4Phagocytosis, step by step

1. The phagocyte is attracted by chemicals released at the infected site and moves towards the bacterium.
2. It attaches to the bacterium.
3. Its cell membrane flows around the bacterium and encloses it in a vacuole: the bacterium has been engulfed.
4. Lysosomes fuse with the vacuole and release digestive enzymes into it.
5. The bacterium is digested, and the harmless remains are released from the cell.

Worked example: a thorn in the foot

Uwase steps on a thorn while working in the garden. It breaks her skin and carries soil bacteria into the wound. Trace her defences.

1. First line breached. The thorn has made a hole in the skin, so the main barrier has failed at that spot. Blood clots within minutes and seals the hole.
2. Inflammation (minutes to hours). Damaged cells release histamine. The area around the wound becomes red, warm, swollen and painful.
3. Phagocytosis (hours). Neutrophils squeeze out of the leaky capillaries and engulf the bacteria. Macrophages follow and clear away debris. If many neutrophils die, a little pus forms.
4. If the infection spreads. Chemical messengers cause a fever, and the adaptive system (the third line) is activated to make specific defences.
5. What Uwase should do. Wash the wound with clean water and soap and cover it. She should go to a health centre if red streaks spread from the wound, if she develops a fever, or if her tetanus vaccination is not up to date, because tetanus bacteria live in soil.

Practice