Core Framework & Diagram The Discovery of T Cells and B Cells
July 24, 20261 Min Read The Accidental Discovery of NK Cells
July 24, 2026Scientists spent seventy years figuring out that the immune system has two completely different types of lymphocytes
—— Thymus, bursa of Fabricius, and those chickens whose organs were cut out: a fascinating and poignant chapter in scientific history.
I. The thymus: considered a vestigial organ for a hundred years
The thymus was a confusing organ for nineteenth-century anatomists. It is quite large in infants and young children, then gradually shrinks and is replaced by fat as age increases. This 'regression' pattern made many scientists think it was a 'vestigial organ' — like the human coccyx, a useless vestige of evolution. For a hundred years, medical mainstream consensus held: the thymus has no particularly important function. This understanding wasn't shattered until 1961. The person who broke it was a 26-year-old Australian PhD student, Jacques Miller. Miller was at the Chester Beatty Research Institute in London, studying mouse leukemia. He noticed that leukemia virus would first infect the thymus, then spread elsewhere. His plan: remove the thymus — perhaps the virus couldn't induce leukemia that way. He had absolutely no expectation of discovering anything else.
He removed the thymus from newborn mice, then saw something he completely hadn't anticipated. Those thymus-free mice grew well, looking completely healthy. But when he gave them skin from other mouse strains — skin that should normally be rejected by immune response — it wasn't rejected. Then he injected these mice with bacteria, and they had almost no ability to resist infection, dying quickly. Miller realized: the thymus is crucial for normal immune system function.
2. A batch of chickens sold to the wrong experiment, and the discovery of B cells
The B cell storyline begins with a batch of innocent chickens and a young student originally researching a completely different problem. In 1955, Bruce Glick, a graduate student at Ohio State University, was studying the bursa of Fabricius in chickens — a pouch-like organ near the chicken's cloaca, whose function nobody at the time knew. Glick was studying it purely out of curiosity. For his experiments, he removed the bursa from a batch of chicks, then sold these chickens to another student in the same lab, Timothy Chang, for a completely unrelated experiment — testing chickens' immune response to Salmonella.
These chickens produced almost no antibodies against Salmonella; a batch died. Chang investigated at length before discovering that these chickens were 'the batch Glick had operated on to remove the bursa.' Glick and Chang realized this was no coincidence. They published their findings: the bursa of Fabricius is essential for antibody production. This was B cells' starting point — from a student who had 'incidentally removed an organ that nobody knew what it did' as an accidental discovery.
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The B cell discovery started from a batch of chickens 'sold to the wrong experiment.' Bruce Glick removed chickens' bursa of Fabricius; those chickens happened to be used in an antibody experiment, accidentally revealing the bursa's essential role in antibody production. Accidents in science are never truly random — they fall into observant eyes. |
3. The letter 'B' actually has two valid origins
B cells' 'B' officially stands for Bursa of Fabricius (the bursa, where B cells develop in chickens). But after the discovery, researchers quickly realized that humans don't have a bursa of Fabricius — so where do human B cells develop? The answer: bone marrow (Bone Marrow). Two words, both starting with B: Bursa, Bone marrow. This is a completely accidental double coincidence in scientific history. This double meaning makes the name 'B cell' applicable to both human and non-human animals, and gives this story an extra layer of charming serendipity.
4. The 'nude mouse': a defect that became the most valuable tool
In 1966, British zoologist Norman Isaacs discovered a strange mutation in a batch of mice: these mice had no thymus and no body hair (the same gene mutation affecting both systems) — their skin was pink and bare, earning them the name 'nude mice.' Nude mice have almost no T cells, because without a thymus, T cells cannot develop normally. This makes them have almost no rejection capacity for foreign cells and foreign tissue. This was exactly what researchers needed: a mouse that barely rejects foreign grafts, which could be used to transplant human tumor cells, study cancer biology, and test new treatments. Nude mice, in the decades that followed, became one of the most commonly used animal models in cancer research laboratories globally. Almost every cancer treatment tested in animals passed through the nude mouse checkpoint. A 'defect' became the most valuable tool — a particularly wonderful accidental beauty in science.
5. What Miller said in his later years
Jacques Miller, in his eighties, gave multiple interviews. The most frequently asked question was: when he did the thymus removal experiment, did he expect to discover the thymus's immune function? His answer was consistently honest and humble: no. He ran this experiment to study leukemia virus, and had completely not expected to find such a fundamental discovery about the immune system. He said that when he saw thymus-removed mice not rejecting foreign skin, his first reaction was: this result is too strange, did I do something wrong? Then he repeated the experiment. Same result. Then he repeated it more times. Same result again. In science, it is often like this: the most important discoveries, when they first appear, make you doubt your own methods, rather than immediately realizing how important they are. That feeling of 'wait, something's not right here' is sometimes the entrance to major discoveries.
6. Why Miller still hasn't received a Nobel Prize
Miller's discovery is recognized as one of the single most important discoveries in twentieth-century immunology. He received the Lasker Award for Basic Medical Research in 2019 — usually considered a Nobel Prize precursor — fifty-eight years late. Many people think he should long since have received a Nobel Prize, but he has not. Possible reasons include: the Nobel Committee tends to favor work with clear 'molecular mechanism' discoveries, while Miller's discovery was functional (that the thymus is useful for immunity), not mechanistic at the molecular level. Also, Nobel Prizes cannot be awarded posthumously — Miller is still alive, so future award is still possible. Based on multiple interviews, he seems quite calm about this. Scientific history has many such regrets — how important a discovery is, and whether one wins a prize, are sometimes far from perfectly correlated.
7. From 'looking identical under microscope' to today's single-cell sequencing
T cells and B cells look identical under ordinary light microscopes — this fact reflects a deeper lesson: our understanding of cells is always limited by the tools we possess. In the 1960s, distinguishing T cells from B cells required carefully designed animal experiments — remove the thymus, remove the bursa, see which immune function disappears. In the 1970s to 1980s, monoclonal antibody technology appeared, and scientists could manufacture antibodies that recognize specific cell surface proteins — T cells (CD3+) and B cells (CD19+) could finally be precisely separated from a single blood tube. In the 1990s, flow cytometry became widespread, enabling simultaneous detection of multiple surface proteins — so people discovered that T cells aren't just 'T cells' but include CD4+ helper T cells and CD8+ cytotoxic T cells, and more subtypes.
In the 2010s, single-cell RNA sequencing (scRNA-seq) emerged. Now researchers can simultaneously measure all gene expression states in a single cell — identifying dozens of different T cell states in a tumor, from naive T cells to memory T cells to exhausted T cells, each with its unique gene expression 'fingerprint.' Miller saw from that batch of 'problem mice' that the thymus 'was useful' — but what exactly 'useful' meant required later people, with better tools, to peel back and see clearly layer by layer. Science advances this way: each generation, in the questions left by predecessors, uses the best available tools of the time to see one more layer. For readers over forty without medical backgrounds, this history has a practical meaning: when you see 'low lymphocyte proportion' or 'abnormal CD4/CD8 ratio' in a physical examination report, you're seeing the result of sixty years of scientific accumulation — from thymus-removed mice, to flow cytometry, to today's precise immune phenotyping. Every number has behind it a history of people working hard to see clearly 'what this cell is and what it's doing.'
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