Immune system is a system of the body that protects it from all foreign substances from the outside (viruses, bacteria, fungi, etc.) and controls the destruction of failed or outdated cells (for example, it is immunity that protects us from tumour formations). In order for immune cells not to destroy the cells of their own body, there is immunological tolerance.
While plants do their best to scare away those who want to eat them by producing all sorts of complex poisons and toxins, ironically, some of them are now used by humans only because of those very poisons.
One of the co-founders of the United Fruit Company (now Chiquita) was Minor Cooper Keith. He built a railroad in Costa Rica and planted many banana trees along the way to feed the workers. When the government could no longer fund the project, Keith had to come up with something to pay back the huge loan he had received. He decided to start using the newly built railroad to export bananas to the United States.
Although appendix is no longer used by humans for digestive purposes it still contains a high concentration of lymphoid tissue. Lymphoid tissue is essential for the immune system and may help encourage the growth of some types of beneficial gut flora.
A sloth can move slowly, but movement is not his primary defense method. Camouflage is a great way to go undetected. And this is where sloths have become experts over a long period of their evolution.