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I have come across the term 'rescue' in biological contexts a lot recently and despite reading multiple definitions online cannot seem to get my head around what the word means on its own, and what it means in biology generally.

The definition differs depending on the context... 'genetic rescue' is a a mitigation strategy, 'synthetic rescue' is a gene alteration that counteracts the deleterious effects of another gene alteration etc.

Could anybody please explain what this means simply and in a way that could apply to different contexts like, for example, virology.

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"Rescue" in an experimental context means you're capable of undoing some experimental manipulation. It's considered evidence towards identifying a causal mechanism or verifying that your manipulation does what you think it did.

For example, let's say you think secretion of molecule ABC is key towards mounting an immune response to MouseKiller virus. You test this by creating a knockout mouse that lacks the enzyme that synthesizes ABC, let's called it Alphabet Synthase, and find out that all the mice die when exposed to MouseKiller virus.

However, maybe you got it wrong. Maybe ABC isn't the key, but actually Alphabet Synthase is involved in some other pathway independent of ABC. You do an experiment where you produce some ABC in the lab, inject it intravenously to your Alphabet Synthase-knockout mice, and find these mice no longer die when infected with MouseKiller. You can say that you've "rescued" the ability of the mice to survive MouseKiller, and now have better evidence that indeed ABC is the key factor.

Another example of a rescue experiment would be that you think specifically Alphabet Synthase in B-cells is important for defense against MouseKiller. It's not very easy to knock out Alphabet Synthase in B-cells specifically, but you can 1) knock out Alphabet Synthase entirely in some mice, and 2) introduce a new version of Alphabet Synthase specifically in B-cells (for example, using a Cre-dependent expression vector with a B-cell-specific promoter). Experiment (1) shows the knockout is susceptible to death from MouseKiller, experiment (2) shows you can rescue the immune defense if B-cells specifically express Alphabet Synthase.

The precise experimental manipulation described as a "rescue" is completely dependent on the research question being asked: you'll have to read specific papers to know what exactly they are doing as a "rescue".

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I suggest that "rescue" is an informal term used in the place of "complementation" in a genetics context. For example, that wiki page has a sentence that reads:

Another exception is transvection, in which the heterozygous combination of two alleles with mutations in different parts of the gene complement each other to rescue a wild type phenotype.

I would suggest that the complementation interpretation means that rescue could be read as: "the restoration of a wild-type phenotype that is otherwise expected to be altered by some insult (genetic or otherwise)".

For example, ura3 yeast have a ura- phenotype such that they cannot grow without provided uracil. However, this growth defect can be complemented or rescued by a plasmid carrying a functional URA3 gene.

I imagine that the word could be used slightly differently in a variety of contexts as biology is a large field. In genetics at least, I would say that the term is used informally and we should not pretend that it has a very specific meaning.

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