Once a virus enters our cells, it quickly hijacks the cell’s manufacturing functions in order to make millions of copies of itself. These viruses then burst out of our cell, killing the host cell and spilling out into our system infecting other cells along the way. So how is our body able to stop the takeover by these viruses?

Our body has 2 types of immune responses. The first one, called our “innate immune system” is shared by all animals. The second immune response is called our “adaptive immune system” and is found in vertebrates.

Our innate immune system is designed to recognize pathogens such as viruses, bacteria, fungi etc, and have the amazing ability to engulf these pathogens and literally digest them by releasing an enzyme that destroys them. These specialized cells are called “macrophages” – macro meaning “big” and phages meaning “eaters”.

Macrophages develop from monocytes (a specific type of white blood cell). They reside in our tissues just waiting for a pathogen to come along. We have about 2 billion of them living in our bodies. Although this is a fantastic defense mechanism, our macrophages do  not retain any “memory” of previous pathogens. This is where the adaptive immune system comes into play.

The adaptive immune system consists of  T cells of which there are about a trillion. These cells are produced in our bone marrow and there are 3 types of T cells: killer T cells, helper T cells and regulatory T cells each performing specialized tasks in the immune response. 

The key difference between the innate and adaptive immune response is in the speed in which they are able to respond to a pathogen. The innate system responds almost immediately because macrophages reside in the billions throughout our body. The adaptive immune system takes up to several weeks to mount a full immune response to a  new pathogen, but once done, it retains a memory of this for life.

That’s why there’s about a 2 week lag time between getting vaccinated from H1N1 virus and developing a full immune response – same with any other type of vaccine that we receive.

So you might be wondering why we need to get annual flu vaccines if our bodies are so good at remembering previous pathogens. The answer is simple – some viruses, like the influenza virus, mutate so quickly that they become unrecognizable the following year.

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