The Second Line of Defence

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The Second Line of Defence

The second line of defence is also a non-specific response (i.e. the response is the same for any pathogen).

It is a 3-pronged attack on any microbes that have survived the first line of defence...

(Yes, this is good!)

Inflammation happens because cells damaged by invading pathogens and particular white blood cells release 'alarm' chemicals which makes blood vessels enlarge (vasodilate) and the capillaries more 'leaky'.

This means that:

  1. More blood is coming to the site of the infection, bringing with it more white blood cells of the immune system
  2. 2. Then, the white blood cells are let out of the blood capillaries and into the affected tissue.

This extra blood makes the area red (as more blood means that the area looks red) and swollen (more blood and liquid leaving the blood and entering the tissue fluid surrounding the body cells).

The area will also become hot (as the extra blood is also carrying heat with it) and painful (because the tissues will be swollen with the blood).

Inflammation attracts white blood cells to the area.

The three types of white blood cell you need to know for your exam are neutrophils, macrophages (these are both phagocytes, which are engulfing cells), and lymphocytes.

The phagocytes (for example a neutrophil), having squeezed through the capillary wall and into the infected tissue, engulf and digest offending bacteria as shown in the following diagram...

  1. The bacteria will be attracted to the membrane of the neutrophil.

    membrane of the neutrophil
  2. Phagocytosis. The neutrophil will engulf the bacteria.

  3. Once in the neutrophil, lysosomes (vesicles containing digestive enzymes) will form and make their way towards the phagosome containing the bacteria.

  4. The lysosomes will fuse with the phagosome.

  5. Now the bacteria will be killed and digested by enzymes.

    digested by enzymes

The lymphocytes will also kill bacteria. However, some bacteria may escape by having a protective cell wall or capsule.

(Note: A good example to remember of a bacterium with a capsule is the bacterium that causes tuberculosis.)

As revolting as pus may be, it is in fact a sign that your immune system is doing what it is designed to do.

Pus is millions of dead immune cells that have previously migrated to the site of the infection and engulfed the pathogens.

Other than direct 'hand-to-hand' combat, some killing is done at a distance.

Macrophages make proteins that act in two ways:

  1. They can punch holes in the bacteria and parasites so that they die.
  2. Or the proteins can stick to the outside of the bacteria to make them more appealing for the phagocytes to eat!

If a virus or an intracellular parasite (one that lives inside a cell) has invaded a cell, the cell will make a chemical called interferon. Interferon ultimately prevents that cell from making molecules that the pathogen would need to survive.

Although the second line of defence is very powerful, it does have a some weaknesses:

  1. It can't deal completely with any one particular micro-organism (some pathogens will nearly always survive this attack).
  2. It can not remember past infections.

This is why a third line of defence is needed (the next Learn-it is about the third line of defence).