Where To Stream Cells at Work! Season 1 Episode 5

The body has been invaded by a mass of cedar pollen. Rushing to the scene so that he can eliminate the cedar pollen, White Blood Cell (Neutrophil) runs into Memory Cell. Memory Cell is a Lymphocyte who retains the immune system's memories. Seeing what's actually going on, Memory Cell is strangely flustered: "What the heck? This is just like the legend". What "legend" is Memory Cell talking about?