Our immune system has two main categories: innate vs. adaptive. Innate immunity refers to the defense mechanisms that we are born with. These are our first lines of defense against pathogens and are nonspecific.
Depending on the threat, our innate immune response can be chemical, physical, or cellular. Whenever we are exposed to a pathogen, our innate immune response kicks in to prevent the spread of the pathogen throughout our body.
In contrast, adaptive immunity, also called acquired immunity, is specific to the pathogen introduced. Adaptive immunity is acquired once a person has been exposed to a disease. It is the second line of defense and is only found in vertebrates.
The adaptive immune system depends on the clonal expansion of lymphocytes. There is a rapid increase of T and B lymphocytes that produce clones having the same antigen receptor as their parent. These millions of lymphocytes attack the specific pathogen that triggers them.
If the innate immune response is enough to suppress the pathogen, then the adaptive immune response does not come into play.