Immunology is the study of the immune system. It's medical research includes all the cells, chemicals, and organs that defend the body against disease. Viruses are not self-sufficient and must invade living cells, using that cells own machinery in order to reproduce. A virus must invade, destroy, and occupy another cell to live. Fungi is a life form slightly more advanced than bacteria but not as developed as plants, and include molds, rusts, mildews and yeast.
Parasites are organisms that feed, grow and take shelter in other organisms acting as the host.
The immune system must identify and destroy these foreign invaders. The white blood cells are the back bone of the immune system. There are many different kinds of white blood cells that have specific jobs to perform. Cells from the spleen, lymph nodes, liver, bone marrow, lungs and intestines are also part of the immune system and help to dispose of pollutants in the blood. The immune system is able to identify intruding organisms as being foreign because the protein chain, antigen, of the organism is different from self cells.