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Innate immunity: the first line of defence

The views expressed are those of the author and are not necessarily those of Scientific American.


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The very first line of defence against any invasion of the human body is a set of physical barriers between the inside of the body and the outer world. Defence systems like the skin, tears and the stomach lining might not sound very impressive until you start to think of what happens when they don’t function. Damage to the skin, such as burns or cuts, very quickly leads to complications and infections that before modern-day antibiotics could easily prove fatal.

Once the outer line of defence is breached, the body very quickly starts up its first major attack on the invading substances. This is called innate immunity, and is made up of a collection of white blood cells along with groups of chemical messengers. Innate immunity is activated by the recognition of a set of molecules that are found only on invading substances rather than on cells of the body. Any sign of things such as bacterial lipopolysaccharide, double-stranded RNA and bacterial flagellin will trigger an inflammatory response.

Scanning electron microscope of blood cells used in the innate immune response. Red blood cells are the smooth ones with the dent in the middle, white blood cells are round and knobbly. By Bruce Wetzel and Harry Schaefer.

The chemical messengers released once the invading pathogen has been sighted include histamine, leukotrienes and prostoglandins. Overall they cause the blood vessels to dilate which brings more white blood cells to the scene. They also cause fluid to leak out of the blood vessels – this contains proteins that wall off the injured area. This leads to many of the symptoms of inflammation: redness, swelling and heat at the site of the infection. These circulating chemicals also activate the white blood cells of the innate immune system, turning them from circulating blobs into pathogen killing machines.

The first cells on the scene are white blood cells called neutrophils. These cells are able to engulf any non-human material and break it down using a cocktail of destructive enzymes. They also release another load of chemicals to summon more white blood cells to the scene. The video below is one of my favourite short clips, and shows a neutrophil hunting down and engulfing a bacteria.

Neutrophils have a very short life-span once activated and very quickly die. These dead cells form pus, which can be eaten up by the macrophages; larger and hardier white blood cells which arrive soon after the neutrophils to tidy away the damage. As well as white blood cells, the body also activates a cascade of protein cleavage reactions known as the complement system. The complement system massively amplifying the signal for infection and can also aid in fighting pathogens. Complement proteins can break into bacterial cell walls, clump together foreign bodies, and attract more macrophages and neutrophils.

While the innate immune response is tackling the invading pathogen, something slightly more sneaky is also going on. White blood cells called dendritic cells will engulf pieces of the invading organism and ferry them to the adaptive immune system command centres in the lymph nodes. It is here that the body plans the most successful and impressive attack on pathogens; producing tailor-made molecules to hunt down and destroy the attacker.

Credit link for image 1.

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. tuned 10:51 am 03/16/2014

    How the heck does the bacteria know to evade the white blood cell, which way to go?
    How does it move so well?
    Where does it keep it’s plans for world domination?

    Link to this
  2. 2. ironjustice 1:20 pm 03/16/2014

    Are you quite sure white blood cells are the ‘first’ line of defense? What about lactoferrin? It resides, is always there in our mucous, so wouldn’t it be considered, technically, the ‘first’ line of defense? Lactoferrin binds up iron when there is a ‘breach’, red blood cell lysis, when it breaks up, such as at a skin break, bleeding ulcer, bruise, etc. It floats around in our mucous, in wait to pick up iron if and when iron becomes ‘labile’, free, at which point the iron could used by the invader in order to propogate and proliferate, so, lactoferrin would be the ‘big dog’ in this scenario?
    “Lactoferrin as a natural immune modulator”

    Link to this
  3. 3. S.E. Gould in reply to S.E. Gould 12:48 pm 03/17/2014

    Thanks for the comments!
    @tuned: The bacteria can sense the blood cell behind it and is just trying to move ‘away’ as fast as possible. It isn’t headed in any particular direction. Bacterial movement is carried out by the flagella – long tentacle-like things which turn in a corkscrew motion together to propel the cell forwards. As for world domination you could argue that the bacteria have got there already. They live all over the world, in huge numbers, and are able to evolve defences against every antimicrobial that gets thrown at them.

    @ironjustice: The innate immune response includes barriers (such as skin, tears and mucus layers in the stomach) as well as white blood cells and platelets. Each of these elements could have several blog posts of their own! I’ve focused mainly on the white blood cells here as they are the components most people study and learn about. I may write a post on mucus, or lactoferrin, in the future.

    Link to this
  4. 4. kneedham 2:11 pm 03/17/2014

    You spelled “complement” wrong!

    Link to this
  5. 5. S.E. Gould in reply to S.E. Gould 2:15 pm 03/17/2014

    Ooops! Should all be corrected now. It is a very polite protein cascade!

    Link to this
  6. 6. larkalt 4:22 am 03/18/2014

    The dendritic cells also play a part in a late phase allergic reaction. In some allergic people, the dendritic cells express the high-affinity IgE receptor, so these cells engulf IgE-antigen complexes and present them to T cells.
    This reaction can be extremely sensitive.

    Link to this

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