Final Answer:
Immediately after activated B cells have migrated to primary follicles, they become proliferating centroblasts.
Step-by-step explanation:
Upon activation, B cells undergo a complex process known as the germinal center reaction, which takes place within primary follicles of secondary lymphoid organs such as lymph nodes and spleen. In this context, the correct answer is that activated B cells become proliferating centroblasts. After migrating to primary follicles, B cells enter the germinal center, a specialized microenvironment where they undergo intense proliferation and genetic diversification.
Within the germinal center, B cells differentiate into centroblasts, which are rapidly dividing cells. These centroblasts then undergo somatic hypermutation, a process that introduces random mutations into the genes encoding immunoglobulins.
This diversification increases the specificity of the antibodies produced by the B cells. The centroblasts subsequently undergo selection processes, with those cells expressing high-affinity antibodies receiving survival signals, while those with lower affinity undergo apoptosis.
Therefore, the designation of activated B cells transforming into proliferating centroblasts accurately reflects the critical stage of B cell activation within the germinal center, highlighting the dynamic and intricate processes involved in the generation of high-affinity antibodies during the immune response.