The specific immune response, both from T lymphocytes (responsible for cellular immunity) and B lymphocytes (responsible for antibody production), takes place in the so-called secondary organs of the immune system, which include the lymph nodes. After immunisation, antigen-presenting cells residing in the different layers of the skin capture the antigen and must migrate to the nearest lymph nodes. There, they begin to present the antigen until they find a B cell that recognises it, activates and begins to produce antibodies, ultimately generating memory cells that will activate upon a second encounter with the same antigen.

The Australian study has shown that re-immunisation in the same area where the first contact with the antigen occurred leads to a faster and more effective immune response. The reason is that the cells residing in the lymph nodes in the area, especially memory B cells, would have a greater response capacity because they are in a pre-sensitised environment that has already responded to that antigen.

The elegant study not only demonstrates the dynamics of the response in animal models, but also verifies it in a group of volunteers who underwent vaccination against SARS-CoV-2 (COVID-19). Although people who were re-immunised in the same arm responded more quickly and effectively than those immunised in the contralateral arm, this difference tended to neutralise over time. Therefore, while contralateral immunisation will also be effective in the long term, this discovery may be important in a pandemic setting, where rapid and powerful protection of the population is required.

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