Locaion on a temperate phage chromosome that is responsible for maintaining the lysogenic state and, as a consequence, effecting immunity against superinfecting phages.
Two regulatory proteins are encoded within the immunity region of phage λ, the canonical temperate phage, as well as six sites to which these proteins, CI and Cro, bind.
The presence of CI along with absence of Cro during infection maintains the lysogenic cycle. Alternatively, the presence of Cro along with the absence of CI is associated with the lytic cycle. During the lytic-lysogeny decision, an intermediate state exists, one that can be tipped in either direction, CI only or Cro only, depending on the relative prevalence of these two proteins. Protein CI in particular serves as the repressor protein in λ.
An enormous amount of detail is known of the phage λ immunity region with numerous reviews in existence. Relatively early in the process of characterizing the λ immunity region, by contrast, the glossary from Hershey (1971), p. 775, defined "immunity region" in this manner: "The segment of a temperate phage chromosome responsible for immunity. More specifically in λ, the regions called imm434 and imm21, consisting of λ DNA that cannot be crossed into a hybrid having the immunity of phage 434 or 21, respectively."
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