What is the first step in making monoclonal antibodies artificially from mice?

Injecting the antigen into the mouse is indeed the first critical step in the production of monoclonal antibodies. This process begins with the immunization of the mouse, where a specific antigen is introduced to stimulate the immune response. The mouse's immune system then generates a diverse array of B cells, which are responsible for producing antibodies against the injected antigen.

Following this initial step, B cells that specifically recognize and bind to the antigen of interest can later be harvested and fused with myeloma cells to create hybridoma cells, which can then be cultured to produce large quantities of the desired antibody. Therefore, the injection of the antigen sets the entire process in motion, making it a fundamental first step in the development of monoclonal antibodies.