How Bacteria Train Our Immune System

How Bacteria Train Our Immune System

When I was growing up, I learned to fear microorganisms. As a child I remember always being concerned about catching a sore throat because the usual treatment was a five-day course of penicillin shots in my backside, twice a day. After more than a couple of sore throats and multiple penicillin shots, I feared being around people with sore throats or colds because I knew that I would be “tortured” again with those shots. I learned to fear tiny, unseen bugs.

As I got older, I saw this fear extended to the population in general in the form of “antimicrobial soaps” and “antimicrobial surfaces.” It seemed that humanity needed to get rid of all bugs, or else we would be subject to torture similar to what I experienced with my penicillin shots. This scenario raises the question: Why would a good God create viruses or bacteria that have the potential to cause serious diseases?

Intimate Bacterial Friends

In the last decade scientific discoveries have shed light on the positive role that these microorganisms play in the human body and how they are important to our health. In this article I will focus on bacterial cells (although there are also viruses and protozoa associated with the human body whose roles are still not completely understood) and their benefits to humans.

The human body is composed of about 30×1012 (30 trillion) cells. The vast majority of these cells are the red cells that make up our blood. In comparison, about 38 trillion bacterial cells line our body.1 This means that there are more bacterial cells than human cells in a human being. Such cells line our skin, mouths, and gut, and are also found in parts of our respiratory and genitourinary systems. By far, most of these cells live in the gut and are known as the human microbiota. These bacterial cells have unique DNA and they produce a different array of proteins than that of the human body. Scientists estimate that 90% of the genes in a human being are derived from the human microbiota, and they comprise what’s called the human microbiome. These bacteria and their genes help protect the human body against threats, provide the body with chemicals essential for adequate functioning, and play many roles in human metabolism that researchers are starting to discover.

Help Begins Early

A human’s first encounter with bacterial cells happens as soon as we are born. When a baby passes through the birth canal, he or she is exposed to the bacteria that line the genital tract and skin of the mother. The bacteria will start covering the baby’s skin as well as his mouth and respiratory tract. When the baby is fed, she swallows bacteria that will eventually establish their home in the gut. Humans are largely unaware of this silent process, but if we look closer we will find that it is crucial for the adequate function of the immune system.

Probably the most complex system in our bodies, the immune system reveals exquisite design. It includes multiple different cells that play a unique role in the crucial task of differentiating between elements that will be beneficial or harmful for the body. We can think of the immune system as our defending army. Humans are born with the basic foundation of the immune system, but its design and function are calibrated by the bacteria that will come in contact with the human body in the first years of life. In other words, the army needs to be trained.2

A baby’s immune system is set up at birth in a bacterial tolerance mode. This permissiveness allows for the careful recognition of bacteria in their newly encountered environment. Through this training process, the immune system will learn to discriminate between “good” and “bad” bacteria (also called pathogens). A tradeoff results as young children have a higher risk of death from infectious diseases than older children. But this risk is attenuated by other factors, including breastfeeding. Here the baby receives antibodies and immune cells that help fend off those dangers. Breastfeeding also produces chemicals that promote the growth of the right bacteria and inhibit the growth of pathogenic bacteria. This permissiveness stops around the age of 3 years, after which the immune system will have finished its training and will be fully deployed for the rest of the person’s life. It will not stop learning about new threats, but its response will reflect the training process.

Calibrated and Designed for Human Benefit

During this training process, the immune system calibrates its responses, which produces adequate activation and deactivation pathways. If thought of as a combat analogy, a defending army needs to know when to attack, how strongly to attack, which weapons to use, and when to stop attacking. Deficiencies in the training process may produce an immune system in a persistently attacking mode, which manifests in the body as autoimmune diseases (Lupus, Crohn’s disease, rheumatoid arthritis) or an immune system that overreacts to environmental elements, which we can see as anaphylactic reactions.

The human immune system exhibits unique foresight in its design. This design not only anticipates encounters with elements that must be identified as threats or nonthreats, but also it anticipates that some elements will need to be embraced and protected for the adequate functioning of the human body. These encounters will also end up calibrating the immune response in a way that maximizes the effectiveness of its responses for years to come.

Such elegant calibration brings to mind the words of the psalmist that we are “fearfully and wonderfully made” (Psalm 139:14). Part of that makeup includes a wide collection of microorganisms that are associated with our bodies. These efficient defenders remind us that the more we discover about microorganisms, the more we can appreciate God’s goodness and love for human beings.

  1. Ron Sender, Shai Fuchs, Ron Milo, “Revised Estimates for the Number of Human and Bacteria Cells in the Body,” PLoS Biology 14, no. 8 (August 19, 2016): e1002533, doi:10.1371/journal.pbio.1002533.
  2. Yasmine Belkaid and Timothy W. Hand, “Role of Microbiota in Immunity and Inflamma-tion,” Cell 157, no. 1 (March 27, 2014): 121–41, doi:10.1016/j.cell.2014.03.011.