Figure 1.

Innate memory in invertebrates and vertebrates. (A) Infections or stressors can prime the innate immune system so that, after a phase of extinction of the response, it will respond more potently to a subsequent challenge with the same or a different stimulus. In invertebrates, this is defined as a recall response, while in vertebrates it is called potentiation or trained immunity (red line). Only in vertebrates, the secondary response can be less intense than the first one, a phenomenon known as tolerance (green line). (B) In invertebrates, a second challenge in primed animals can lead to an immune shift, i.e., the shift from a type of response (dotted line) to a different, more efficient one (solid line). (C) In invertebrates, priming could result in a medium- or long-term immune activation state, which can further increase upon challenge. This is identified as sustained or unique response. Adapted from Coustau et al. (48) and Pradeu and Du Pasquier (15).