And a pathogen is any type of agent, it can be living or non living that brings harm to the cells of our body. So a bacterial cell, a virus, a chemical that can harm our cells, all these different things are considered to be pathogens. Now, for a pathogen to actually cause damage, cause harm to the cells of our body, these pathogens must enter our body. And the most common ways by which pathogens enter our body is through the simple process of breathing when we exchange air inside our lungs that can bring different types of pathogens into our body. Pathogens can also enter our body through the food that we eat, through the water that we drink or through some type of cut that is found on our skin. Now, in order for our immune system to actually be effective and efficient, it must have a way to actually differentiate, to actually distinguish between its own cells of the body and these foreign pathogens that can cause harm to those cells of the body.