Bacterium (example)
Bacterium (example).




Virus infecting cell
Virus infecting cell.


The immune system responds to bacteria and viruses in a very complex way: it recognizes unique molecules (antigensAntigenA foreign substance in the body that triggers the production of antibodies.) from bacteria and viruses and produces antibodies (a type of protein) and special white blood cells called lymphocytes that mark the antigens for destruction.

During the primary immune responseImmune responseThe body's defense against foreign objects or organisms, such as bacteria, viruses or transplanted organs or tissue. to the first encounter with a specific pathogen, some lymphocytes called memory cells develop with the ability to confer long-lasting immunityImmunityThe body's response mechanism for fighting against bacteria, viruses and other foreign substances. If a cell or tissue (such as bacteria or a transplanted organ) is recognized as not belonging to the body, the immune system will act against the "invader." The immune system is the body's way to fight external invasions. to that pathogen, often for life. These memory cells recognize antigens on the pathogens they have encountered before, triggering the immune system to respond faster and more effectively than on the first exposure.

Primary and secondary immune response
Primary and secondary immune response.

Source 1:, Source 2:

The graph below compares the primary and secondary immune responses to the same pathogen. The secondary response may eliminate the pathogens before any damage occurs.59

Primary and secondary immune responses to the same pathogen
Primary and secondary immune responses to the same pathogen

Key point

Immunization triggers an immune system response by which the vaccinee develops long-term protection (immunity) that would normally follow recovery from many naturally occurring infections.