https://doi.org/10.1351/goldbook.13601
Immune response to the first encounter with an antigen, characterized by slow production of antibody molecules and the priming of lymphoid tissue in readiness for the production of a secondary immune response on subsequent challenge with the same antigen.
Note: The primary response is generally weak, has a long induction phase or lag period, consists primarily of the release of immunoglobulin M (IgM) antibodies, and generates immunologic memory.