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.
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.