Add avs and trigger

Adding prewriting pages on Avs and Phage triggers

content/2.general-concepts/3.phage_trigger.md

+---
+title: Defense system triggers
+contributors: 
+    - Avigail Stokar-Avihail
+---
+
 # How anti-phage systems sense invading phages
 Upon phage infection, the bacterial immune system senses a specific phage component or modification that the phage exerts on the cell to elicit the bacterial immune response. Understanding how bacteria sense phage infection is a fundamental question, which remains unanswered for the majority of recently discovered immune systems. There are dozens of cases in which the mechanism of immunity has been elucidated, but the phage trigger remains elusive. Understanding how antiphage systems are activated is key for a full understanding of bacterial immunity and for repurposing them as molecular tools as has been done for restriction enzymes and CRISPR-Cas.