What is the second line of defence?

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What is the second line of defence?

Before we start let’s re-cap some key terminology:

-pathogen: a microorganism causing disease or illness to its host

-lymphocyte: a small white blood cell involved in immunity. There are two major types; B and T.

-phagocyte: a large white blood cell that ingests foreign particles

-inflammation: a response to tissue injury characterised by swelling, heat, pain and redness

-lysosomes: vesicles containing digestive enzymes

Description: http://www.nature.com/nrd/journal/v7/n9/images/nrd2673-i1.jpg

The three-pronged fork

The second-line of defence is like this fork. There are three mechanisms that work together to produce this response. We will cover the first step.

1)      Inflammation

‘Alarm’ signals are sent which does two things; (i) makes the blood vessels vasodilate which means they get bigger as they fill with blood. As more blood pools to this area of tissue damage there will be redness and swelling (ii) makes the capillary walls ‘leaky’ which allows the white blood cells to move out of the blood, through the walls and into the tissue

The advantage of inflammation is that it allows the needed white blood cells to pool into this area.

2)      Phagocytosis

There are two important phagocytes that you should be familiar with; neutrophils and macrophages. Phagocytosis is the process in which the phagocyte, after leaving the bloodstream and entering the infected tissue site, will digest the pathogen (think of it as a pacman!)

-the bacteria are attracted to the surface of the phagocyte

-the phagocyte will engulf the bacterium

-the engulfed bacterium is now called a phagosome

-lysosomes fuse with the phagosome and digest it

-the bacteria has been killed and digested

3)      Macrophages

They also promote the destruction of pathogens ‘from a distance’ by making proteins:

-they can punch holes into the cell wall of pathogens causing them to die by osmolysis

-they can make proteins which stick to the pathogens and acts as signals attracting more white blood cells to come and destroy them


Although the second line of defence is a much needed response, can you think of any flaws to the system?

-is it able to remember past infections?

-are some pathogens able to escape this response?

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