Immune System: What Happens During an Infection?
Primary immune response
Our immune system generates a primary immune response the first time it is exposed to a pathogen. The response takes several days to develop. It also takes some time to overcome the infection. During this period, some long-lasting cells will be made. These cells have the ability to “recognize” the pathogen. They are called memory cells. They remain at low levels and travel throughout the body after the infection is resolved. So, if you are exposed to the same pathogen again, your immune system is better prepared.
Secondary immune response
Memory cells are an important part of secondary immune responses. These are the basis for immune responses to second (or third or fourth, etc.) exposures to a pathogen. Secondary immune responses occur more quickly because of the presence of memory cells. Secondary immune responses are also more specialized because the memory cells are specific for the pathogen. Whereas it can take up to five days for antibodies from primary immune responses to be detected, antibodies from secondary responses can be measured within three days. The result is that you are typically less ill, if at all, and more quickly feeling back to yourself.
Find a more detailed discussion on the VEC website.
Reviewed by Paul A. Offit, MD, on Nov. 20, 2025