The innate immune system is a system bestowed unto a fetus from the day we were born.  This immune system is the first and second line of defense that protects one's body from all forms of pathogens using various physical barriers.

The skin and mucous membranes act as shields and barriers for pathogens.  The skin acts as one’s wall such that a pathogen can’t enter into their body.  For the areas where skin cells aren’t present such as the nose, mouth and eyes the mucous membranes are there to stop the pathogens from entering any deeper into the body.  Mucous membranes protect the body in two ways, the first of which is through its structure, these membranes are made up of epithelial tissue which stops pathogens from entering further.  The second is through the secretion of mucus, this mucus prevents movement between the pathogens, keeps its surroundings moist and has instituted antibiotics called defisins which fight the pathogens.

These proteins are seemingly passive and don’t do much other than mindlessly floating in the midst of our bodies as do other mindless substances, that is until they are activated and change their shape.  Complement proteins when activated can do serious harm to pathogens and the proteins activate each other much like a burning match lighting up other matches. This chain starts with the c3 protein, this protein is activated randomly, through other complement proteins or through antibodies.  When activated the protein splits into two, c3a and c3b.  c3b seeks nearby viruses or bacteria and attaches onto them, when attached it changes its shape, in its new shape it is now able to grab onto other nearby proteins, activate those proteins and it adds other proteins on top of itself forming a small cascade. Finally it transforms itself into a recruiting platform known as c3 convertase, this platform activates other c3 proteins and starts the cycle anew.  When thousands of proteins cover the pathogen it gets immensely crippled and becomes helpless.

Now we look at the other part of c3, the c3a protein.  C3a acts as a distress beacon or a flare of sorts, thousands of these tiny proteins flee from the site of battle and find passive immune cells such as phagocytes and lead these cells directly towards the battle.  The more alarm c3a  proteins that these immune cells encounter the more aggressive they become.  Through this complement guides these cells towards exactly where they need to be.

The last way of the complement system defending us is through directly attacking the pathogen. Soon a new cascade will begin in the c3 convertase, the recruitment platform changes shape again and begins to recruit other proteins to create what's called a “Membrane attack complex”.  Piece by piece new proteins shaped like spears begin to pierce the membrane of the pathogen until a hole is ripped, fluids rush out of the bacteria because of dispersion and the pathogen “bleeds out”.