Human botfly

Know your enemy

Our bodies are homes to millions of organisms

Millions of microbes have made your body their habitat and most of them will never do you any harm. However, some can be pathogenic, which means that they cause disease. Human pathogens include some bacteriaviruses and fungi, as well as parasites such as tapeworms and flukes, and protozoa like Plasmodium, which causes malaria.

Some microbes can be good for us. It’s known that the gut microbiota, the bacteria in our gastrointestinal tract, exist in a mutualistic relationship with us – one where both parties benefit. But it’s not clear how the immune system views these cells. Recent studies suggest that newly discovered populations of immune cells may help train the immune system to tolerate beneficial bacteria.

  Organisms included Number of cells Presence of mitochondria Type of immune system Type of reproduction
Eukaryotes Animals, plants, fungi Often, but not always, multicellular Cells use mitochondria to release energy Some multicellular eukaryotes have sophisticated circulatory, nervous and immune systems Sexual or asexual reproduction
Prokaryotes Bacteria and cyanobacteria Always single-celled No mitochondria Basic immune responses Usually, but not always, asexual reproduction
Viruses Do not fit many of the criteria for life No cells, just virus particles called virions (RNA or DNA inside a protein coat) No mitochondria No immune system Viral replication by taking over a host cell


Lead image:

Human botfly.

Geoff Gallice/Flickr CC BY NC


Downloadable resources

About this resource

This resource was first published in ‘Immune System’ in January 2015.

Microbiology, Immunology
Immune System
Education levels:
14–16, 16–19, Continuing professional development