The graph shows the level of antibody in serum following vaccination and a challenge with the same antigen 28 days later.
Study the graphs below and answer the question:

In reference to the graph above, which graph 1 or 2 represent secondary response?

Explanation

Graph. During the primary immune response to the first encounter with a specific pathogen, some lymphocytes called memory cells develop with the ability to confer long-lasting immunity 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 expsure.