We just learned about the type of white blood cell called a Lymphocyte.
There are a few types of lymphocytes, like the NK Cells, also called Natural Killer Cells or large granular lymphocytes (LGL).
These cells float around in the body, and if they find a cell that doesn't look like it's supposed to be part of the body then it lets out some chemicals called cytokines that will destroy the bad cell.
The bad cells might be like a tumor, an infection or a virus, and when the NK Cell releases it's cytokins and the bad cell will be destroyed.
Because these LGLs kill other bad cells they are called natural killers.
(from: wikipedia - natural killer cell)
Kid Facts - Blast from the past: Aqueous Humour
