ABSTRACT
The immune system is comprised of organs, structures, cells, and factors that are directly involved in the defense activity of the host against pathogens (viruses, bacteria, fungi, and parasites) and transformed cells (tumors). Evolution has finely shaped and molded this system producing very different levels of organization, spanning from the very simple and primitive systems of the most elementary multicellular invertebrate organisms, to the complex and highly specialized systems of higher vertebrates.
