Immunologic memory is the most distinctive feature of adaptive immunity, with protective immunization the prototypical example of this powerful phenomenon.
Differences Between Primary and Secondary Responses
The idea of an anamnestic response posits that the second encounter with an antigen is regulated differently from the first. During the processing phase of the primary response, relatively rare antigen-specific B lymphocytes (perhaps 1 in 100,000 B lymphocytes) and T lymphocytes (perhaps 1 in 10,000 T lymphocytes) must come in contact with appropriately presented antigen. Stimulation of these cells from a completely resting and naive state then occurs, a process that requires days. Following the primary response, various events occur that set the stage for a subsequent rapid and robust secondary response:
Following stimulation, lymphocytes divide, dramatically increasing the population of antigen-responsive T and B lymphocytes (clonal expansion). In addition, these cells migrate to other sites of potential encounter with antigen.
Upon removal of antigen, T and B lymphocytes activated during the primary response gradually return to a resting state but are no longer naive. They retain the capacity to become reactivated within 12–24 hours of antigen exposure and thus are termed memory cells.
IgM produced during the primary response is often too large (molecular mass, 900 kDa) to leak passively into a peripheral site. Following antibody-class switching, IgG or other isotypes can leak passively into a site or are already present because they were produced at the site; thus, they can immediately combine with antigen and trigger a very rapid response.
In some cases, such as in mycobacterial infection, low doses of antigen may remain in the node or site, producing a chronic, low-level, continuous antigenic stimulation of T and B lymphocytes.
Excerpted from BCSC 2020-2021 series: Section 9 - Uveitis and Ocular Inflammation. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.