Thinking Beyond Science

Your immune system works around the clock—an invisible battalion guarding you against the microscopic threats that fill your world. Every second, thousands of unseen enemies—germs, pathogens, environmental toxins—test your body’s defenses. From the sneeze cloud on your morning bus ride to the bacteria clinging to your salad leaves, your immune system’s white blood cells are constantly on patrol.

These white blood cells—the soldiers of your body—don’t just fight off invaders; they also heal. When you cut your thumb chopping onions, they rush to the site, sealing the wound and fending off infection. The redness and swelling? That’s your body’s repair team at work.

But, like any powerful defense system, things can go wrong. Sometimes, under the weight of endless environmental stressors, your immune army loses control. It can go into overdrive, attacking not just germs but your own tissues—leading to autoimmune diseases such as rheumatoid arthritis, lupus, multiple sclerosis, and type 1 diabetes. When inflammation goes from protective to destructive, the same energy that saves your life can turn against you.

The Brain: Once Thought Immune to Immunity
For over a century, scientists believed one organ stood above this cellular warfare: the brain. The long-standing medical doctrine was that the brain was “immune-privileged,” protected behind a barrier that kept the immune system out. That meant inflammation couldn’t occur unless the brain was directly attacked—say, by trauma or infections like meningitis.

In this view, the brain’s neurons reigned supreme, orchestrating mood, memory, and cognition, while glial cells acted as their support crew. Among these glial cells were several types with specialized roles: oligodendrocytes helped form the insulating myelin around nerves, astrocytes nurtured neuronal growth, and Schwann cells facilitated communication. But one family of glial cells—microglia—was largely ignored, dismissed as irrelevant to normal brain function.

Microglia: The Brain’s Secret Immune Guardians
That changed when researchers at Mount Sinai began tracing the origins of these mysterious microglia. They discovered that, during early embryonic development, microglia actually share roots with white blood cells. Both emerge from the same family of stem cells, but instead of remaining in the body, microglia migrate into the brain around the ninth day of gestation. Once there, they stay for life.

This revelation reshaped neuroscience. Microglia weren’t just passive helpers—they were the brain’s own immune police, patrolling the neural landscape much like white blood cells patrol the body. When neurons die or unwanted substances appear, microglia spring into action. They identify debris, engulf it, and restore order.

The Parallel Worlds of Immunity
Macrophages—the body’s “big eaters”—perform a similar role outside the brain. They consume pathogens and dead cells marked by complement molecules, maintaining balance within tissues. But when overactivated, they can trigger chronic inflammation and even tissue destruction, as seen in autoimmune disorders.

For years, scientists believed such complement tagging and cell cleanup didn’t happen in the brain. The brain, they thought, was a fortress untouched by immunological activity. Yet, new research suggests otherwise: microglia not only defend but also sculpt the brain. By pruning synapses during development and responding to stressors throughout life, they connect the immune system and the mind in ways we’re only beginning to understand.

The Mind-Body Connection, Reinvented
What began as a simple idea—a clear divide between brain and body—has evolved into something beautifully complex. The immune system doesn’t just defend you; it also influences how you think, feel, and heal. Your microglia, those once-overlooked brain cells, form a hidden bridge between immunity and identity.

The lesson? The boundaries within our bodies are not as rigid as once believed. Health, whether mental or physical, is a conversation—a constant, cellular dialogue between our body’s armies and our brain’s inner guardians.

Source : The Angel and the Assassin: The Tiny Brain Cell That Changed the Course of Medicine by Donna Jackson Nakazawa

Goodreads : https://www.goodreads.com/book/show/45442641-the-angel-and-the-assassin