Journal of Vaccine Research, 2025, Vol.15, No.1, 10-22 http://medscipublisher.com/index.php/jvr 11 2 The Immunological Basis of Long-Term Vaccine-Induced Immunity 2.1 The formation mechanism of memory B cells and long-lived plasma cells The emergence of memory B cells and long-lived plasma cells is the key to maintaining long-term antibody immunity in the human body. When the human body comes into contact with an antigen, B cells will be activated and then undergo changes through germinal center reactions. During this process, B cells will continuously divide and increase in number, and somatic mutations will also occur. Eventually, cells that can better bind to the antigen will be selected. This series of processes generates two important types of cells: memory B cells, which act like a "reserve force" and can respond quickly when encountering the same antigen next time. There are also plasma cells capable of secreting antibodies (Cancer and Tomayko, 2021; Ripperger and Bhattacharya, 2021; Roy et al., 2023). Long-lived plasma cells will run to "sanctuaries" such as bone marrow, where they can survive for several years or even decades, and can continuously produce antibodies without the need for constant antigen stimulation (Slifka and Ahmed, 1998; Akkaya et al., 2019; Ionescu and Urschel, 2019). Memory B cells are not exactly the same. They differ in antibody types, mutation conditions, and distribution locations, and are distributed in the spleen, blood, and body barrier tissues. These cells do not divide normally, have a long survival time and are in a "dormant" state. They will not be awakened until they encounter antigens again. Once activated, they can quickly transform into plasma cells capable of secreting antibodies (Ochsenbein et al., 2000). Scientists are still conducting continuous research on how memory B cells persist and renew over the long term, as well as their relationship with plasma cells (Cancro and Tomayko, 2021; Inoue and Kurosaki, 2023). 2.2 The role of memory T cells in continuous immune protection Although the previous discussion mainly focused on the immunity brought by B cells, T cells, especially memory T cells, are also crucial for vaccines to provide long-term protection. During the germinal center reaction process, memory T cells assist B cells to help form and maintain memory B cells and plasma cells that can better bind to antigens. This auxiliary effect is crucial for memory B cells to smoothly transform into plasma cells after re-exposure to antigens. Only in this way can the human body rapidly and forcefully initiate the second immune response (Ochsenbein et al., 2000; Roy et al., 2023). In addition, memory T cells can also expand the coverage of the immune response and enhance the immune effect, especially when dealing with mutated bacteria. These cells can remain in a "standby" state for a long time. Once the body is reinfected, they will immediately take action. It is precisely because of this ability of memory T cells that many vaccines can exert protective effects for a long time, which also makes up for the deficiency of memory B cells in producing antibodies for defense (Ochsenbein et al., 2000; Roy et al., 2023). 2.3 Affinity maturation and the production of long-term antibodies Affinity maturation is a process that occurs in the germinal center. During this process, the receptors used by B cells to recognize antigens will become more likely to bind to antigens through mutations and screening of the cells themselves. After such changes, the newly generated memory B cells and plasma cells can produce antibodies that bind more closely to antigens. This step is very crucial and plays a decisive role in achieving effective and long-lasting immune protection (Inoue, 2023; Roy et al., 2023). The ultimate effect of affinity maturation directly affects whether the antibody response produced by the body after vaccination is good or not, and how long this response can last. The long-term presence of antibodies is due to the functioning of long-lived plasma cells in the bone marrow. These cells can continuously secrete antibodies without further contact with antigens. They have always been there, ensuring the content of protective antibodies in the body, just like the "first line of defense" against reinfection. The compatibility between mature memory B cells and long-lived plasma cells is fundamental to maintaining the long-term immunity brought by vaccines (Slifka and Ahmed, 1998; Ionescu and Urschel, 2019; Akkaya et al., 2019; Roy et al., 2023).
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