system
Given the following post, respond as if you're commenting on Reddit:
human
How do trees fight off disease?

I'm assuming trees don't have an immune system, so do they have any protection against disease?
gpt
Trees *do* in fact have an immune system, as all plants do! Plant immunity is an innate immune system, while animals have innate and adaptive immune systems.

Plant immunity is, in general terms, concerned with detecting pathogenic proteins and/or pathogen-associated molecular patterns that would signal the presence of a potential plant pathogen. 

Once detected, plants activate a number of processes designed to make the plant tissue inhospitable for a plant pathogen. A classic hallmark of plant immunity is the production of reactive oxygen species (or ROS) that will kill pathogens or drive them to dormancy. This is analogous to a fever in animals.

Another molecular response that occurs in plant immunity is the creation of barriers to the diffusion of pathogens. Deposition of callose - a complex polymer - prevents pathogens from growing or moving freely in the interleaf space. In more serious immune reactions, plant cells around the pathogen will kill themselves in a process known as the hypersensitive response (HR). This denies the pathogen living tissues and nutrients.

Another important barrier that prevents plants getting sick is physical barriers. The bark of a tree is an excellent barrier, much as our skin is, to prevent pathogens from getting anywhere near the important parts of the tree. This is why the vast majority of tree diseases are seen after the tree is damaged by pruning, weather, or by burrowing insects.