View source for Biology:History of phagocytosis

[[File:Neutrophil_with_anthrax_copy.jpg|right|thumb|[[Physics:Scanning electron microscope|Scanning electron micrograph]] of a phagocyte (yellow, right) phagocytosing anthrax bacilli (orange, left)]]
The '''history of phagocytosis''' is an account of the discoveries of cells, known as phagocytes, that are capable of eating other cells or particles, and how that eventually established the science of [[Biology:Immunology|immunology]].<ref>{{Cite journal |last=Tauber |first=A. I. |date=1992 |title=The birth of immunology. III. The fate of the phagocytosis theory |journal=Cellular Immunology |volume=139 |issue=2 |pages=505–530 |doi=10.1016/0008-8749(92)90089-8 |issn=0008-8749 |pmid=1733516|doi-access=free }}</ref><ref name=":12">{{Cite journal |last1=Teti |first1=Giuseppe |last2=Biondo |first2=Carmelo |last3=Beninati |first3=Concetta |date=2016 |title=The Phagocyte, Metchnikoff, and the Foundation of Immunology |journal=Microbiology Spectrum |volume=4 |issue=2 |pages=MCHD-0009-2015 (online) |doi=10.1128/microbiolspec.MCHD-0009-2015 |issn=2165-0497 |pmid=27227301 |doi-access=free}}</ref> Phagocytosis is broadly used in two ways in different organisms, for feeding in unicellular organisms (protists) and for [[Biology:Immune response|immune response]] to [[Biology:Innate immune system|protect the body against infections]] in metazoans.<ref>{{Cite book |last1=Gray |first1=Matthew |last2=Botelho |first2=Roberto J. |title=Phagocytosis and Phagosomes |date=2017 |chapter=Phagocytosis: Hungry, Hungry Cells |chapter-url=https://pubmed.ncbi.nlm.nih.gov/27815869 |series=Methods in Molecular Biology |volume=1519 |pages=1–16 |doi=10.1007/978-1-4939-6581-6_1 |issn=1940-6029 |pmid=27815869|isbn=978-1-4939-6579-3 }}</ref> Although it is found in a variety of organisms with different functions, its fundamental process is [[Biology:Endocytosis|cellular ingestion]] of foreign (external) materials, and thus, is considered as an evolutionary conserved process.<ref>{{Cite journal |last1=Lancaster |first1=Charlene E. |last2=Ho |first2=Cheuk Y. |last3=Hipolito |first3=Victoria E. B. |last4=Botelho |first4=Roberto J. |last5=Terebiznik |first5=Mauricio R. |date=2019 |title=Phagocytosis: what's on the menu? 1 |url=https://pubmed.ncbi.nlm.nih.gov/29791809 |journal=Biochemistry and Cell Biology |volume=97 |issue=1 |pages=21–29 |doi=10.1139/bcb-2018-0008 |issn=1208-6002 |pmid=29791809|s2cid=43942017 }}</ref>

The biological theory and concept, experimental observations and the name, phagocyte ({{etymology|grc|''φαγεῖν'' (phagein)|to eat||''κύτος'' (kytos)|cell}}) were introduced by a Ukrainian zoologist [[Biography:Élie Metchnikoff|Élie Metchnikoff]] in 1883, the moment regarded as the foundation or birth of immunology.<ref>{{Cite journal |last1=Teti |first1=Giuseppe |last2=Biondo |first2=Carmelo |last3=Beninati |first3=Concetta |date=2016 |title=The phagocyte, Metchnikoff, and the foundation of immunology |journal=Microbiology Spectrum |volume=4 |issue=2 |pages=MCHD-0009-2015 (online) |doi=10.1128/microbiolspec.MCHD-0009-2015 |issn=2165-0497 |pmid=27227301 |doi-access=free}}</ref><ref>{{Cite journal |last=Kaufmann |first=Stefan H E |date=2008 |title=Immunology's foundation: the 100-year anniversary of the Nobel Prize to Paul Ehrlich and Elie Metchnikoff |url=http://www.nature.com/articles/ni0708-705 |journal=Nature Immunology |language=en |volume=9 |issue=7 |pages=705–712 |doi=10.1038/ni0708-705 |issn=1529-2908 |pmid=18563076|s2cid=205359637 }}</ref> The discovery of phagocytes and the process of innate immunity earned Metchnikoff the 1908 Nobel Prize in Physiology or Medicine, and the epithet "father of natural immunity".<ref>{{Cite journal |last=Gordon |first=Siamon |date=2008 |title=Elie Metchnikoff: father of natural immunity |journal=European Journal of Immunology |volume=38 |issue=12 |pages=3257–3264 |doi=10.1002/eji.200838855 |issn=1521-4141 |pmid=19039772|s2cid=658489 |doi-access=free }}</ref>

However, the cellular process was known before Metchnikoff's works, but with inconclusive descriptions. The first scientific description was from Albert von Kölliker who in 1849 reported an alga eating a microbe. In 1862, [[Biography:Ernst Haeckel|Ernst Haeckel]] experimentally showed that some blood cells in a slug could ingest external particles.<ref name=":5">{{Citation |last=Stossel |first=Thomas P. |title=Phagocytosis: The Host |chapter=The early history of phagocytosis |date=1999 |chapter-url=https://linkinghub.elsevier.com/retrieve/pii/S187451729980025X |series=Advances in Cellular and Molecular Biology of Membranes and Organelles |volume=5 |pages=3–18 |access-date=2023-04-06 |publisher=Elsevier |language=en |doi=10.1016/s1874-5172(99)80025-x |isbn=978-1-55938-999-0}}</ref> By then evidences were mounting that leucocytes can perform cell eating just like protists, but it was not until Metchnikoff showed that specific leukocytes (in his case macrophages) eat cell that the role of phagocytosis in immunity was realised.<ref>{{Cite journal |last=Cavaillon |first=Jean-Marc |date=2011 |title=The historical milestones in the understanding of leukocyte biology initiated by Elie Metchnikoff |url=https://pubmed.ncbi.nlm.nih.gov/21628329 |journal=Journal of Leukocyte Biology |volume=90 |issue=3 |pages=413–424 |doi=10.1189/jlb.0211094 |issn=1938-3673 |pmid=21628329|s2cid=6804829 }}</ref><ref name=":13">{{Cite journal |last=Gordon |first=Siamon |date=2016 |title=Elie Metchnikoff, the Man and the Myth |journal=Journal of Innate Immunity |volume=8 |issue=3 |pages=223–227 |doi=10.1159/000443331 |issn=1662-8128 |pmc=6738810 |pmid=26836137}}</ref>

== Discovery of cell feeding ==
Phagocytosis was first observed as a process by which unicellular organisms eat their food, usually smaller organisms like protists and bacteria. The earliest definitive account was given by Swiss scientist Albert von Kölliker in 1849.<ref name=":5" /> As he reported in the journal {{Lang|de|Zeitschrift für Wissenschaftliche Zoologie}}'','' Kölliker described the feeding process of an amoeba-like alga, ''[[Biology:Actinophryid|Actinophyrys sol]]'' (a heliozoan). Under microscope, he noticed that the protist engulfed and swallowed (the process now called endocytosis) a small organism, that he named infusoria (a generic name for microbes at the time). Modern translation of his description reads:<blockquote>The creature [infusoria] which is destined for food [i.e., trapped by the spines], gradually reaches the surface of the animal [i.e., ''Actinophyrys''), in particular, the thread that caught it is shortened to nothing, or, as it often happens, once trapped in the thread space, the thread unwinds from around the prey when close together and at the surface of the cell body... The place on the cell surface where the caught animal is, gradually becomes a deeper and deeper pit into which the prey, which is attached everywhere to the cell surface, comes to rest. Now, by continuing to draw in the body wall, the pit gets deeper, and the prey which was previously on the edge of the ''Actinophrys'', disappears completely, and at the same time the catching threads, which still lay with their points against each other, cancel each other out and extend again. Finally, the edges "choke" the pit, so that it is flask-shaped (''flaschenformig'') all sides increasingly merging together, so that the pit completely closes and the prey is completely within the cortical cytoplasm.<ref name=":4">{{Cite book |last=Hallett |first=Maurice B. |title=Molecular and Cellular Biology of Phagocytosis |chapter=A Brief History of Phagocytosis |series=Advances in Experimental Medicine and Biology |date=2020 |chapter-url=https://pubmed.ncbi.nlm.nih.gov/32399823 |volume=1246 |pages=9–42 |doi=10.1007/978-3-030-40406-2_2 |issn=0065-2598 |pmid=32399823|isbn=978-3-030-40405-5 |s2cid=218618570 }}</ref></blockquote>The general process given by Kölliker correlates with modern understanding of phagocytosis as a feeding method. The thread and thread space are [[Biology:Pseudopodia|pseudopodia]], gradually deepening pit is the endocytosis, the ''flaschenformig'' structure is the [[Biology:Phagosome|phagosome]].<ref name=":4" /><ref>{{Cite journal |last1=Shi |first1=Yijing |last2=Queller |first2=David C. |last3=Tian |first3=Yuehui |last4=Zhang |first4=Siyi |last5=Yan |first5=Qingyun |last6=He |first6=Zhili |last7=He |first7=Zhenzhen |last8=Wu |first8=Chenyuan |last9=Wang |first9=Cheng |last10=Shu |first10=Longfei |date=2021 |title=The Ecology and Evolution of Amoeba-Bacterium Interactions |journal=Applied and Environmental Microbiology |volume=87 |issue=2 |pages=e01866–20 |doi=10.1128/AEM.01866-20 |issn=1098-5336 |pmc=7783332 |pmid=33158887|bibcode=2021ApEnM..87E1866S }}</ref><ref>{{Cite journal |last=Jeon |first=K. W. |date=1995 |title=The large, free-living amoebae: wonderful cells for biological studies |url=https://pubmed.ncbi.nlm.nih.gov/7728136 |journal=The Journal of Eukaryotic Microbiology |volume=42 |issue=1 |pages=1–7 |doi=10.1111/j.1550-7408.1995.tb01532.x |issn=1066-5234 |pmid=7728136|s2cid=42349536 }}</ref>

== Discovery of phagocytic immune cells ==
[[File:Die Radiolarien (Rhizopoda radiaria) - eine Monographie (IA dieradiolarienrh03haec).pdf|thumb|Haeckel's monograph in which the ability of blood cell to ingest particles is first reported]]

=== Eosinophils ===
The first demonstration of phagocytosis as a property of leukocytes, the immune cells, was from the German zoologist [[Biography:Ernst Haeckel|Ernst Haeckel]].<ref name=":6">{{Cite journal |last=Cheng |first=Thomas C. |date=1983 |title=The role of lysosomes in molluscan inflammation |url=https://academic.oup.com/icb/article-lookup/doi/10.1093/icb/23.1.129 |journal=American Zoologist |language=en |volume=23 |issue=1 |pages=129–144 |doi=10.1093/icb/23.1.129 |issn=0003-1569}}</ref><ref>{{Citation |last=Cheng |first=Thomas C. |title=Biochemical and Ultrastructural Evidence for the Double Role of Phagocytosis in Molluscs: Defense and Nutrition |date=1977 |url=http://link.springer.com/10.1007/978-1-4615-7299-2_2 |work=Comparative Pathobiology |pages=21–30 |editor-last=Bulla |editor-first=Lee A. |access-date=2023-04-07 |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4615-7299-2_2 |isbn=978-1-4615-7301-2 |editor2-last=Cheng |editor2-first=Thomas C.}}</ref> In 1846, English physician Thomas Wharton Jones had discovered that a group of leucocytes, which he called "granule-cell" (later renamed and identified as [[Biology:Eosinophil|eosinophil]]<ref>{{Cite journal |last1=Koenderman |first1=Leo |last2=Hassani |first2=Marwan |last3=Mukherjee |first3=Manali |last4=Nair |first4=Parameswaran |date=2021 |title=Monitoring eosinophils to guide therapy with biologics in asthma: does the compartment matter? |journal=Allergy |volume=76 |issue=4 |pages=1294–1297 |doi=10.1111/all.14700 |issn=1398-9995 |pmc=8246958 |pmid=33301608}}</ref>), could change shape, the phenomenon later called [[Biology:Amoeboid movement|amoeboid movement]]. Jones studied the bloods of different animals, from invertebrates to mammals,<ref>{{Cite journal |last=Jones |first=Thomas Wharton |date=1846-12-31 |title=V. The blood-corpuscle considered in its different phases of development in the animal series. Memoir II.—Invertebrata |journal=Philosophical Transactions of the Royal Society of London |language=en |volume=136 |pages=89–101 |doi=10.1098/rstl.1846.0006 |s2cid=111214402 |issn=0261-0523|doi-access=free }}</ref><ref>{{Cite journal |last=Jones |first=Thomas Wharton |date=1846-12-31 |title=VI. The blood-corpuscle considered in its different phases of development in the animal series. Memoir III.— Comparison between the blood-corpuscle of the vertebrata and that of the invertebrata |url=https://royalsocietypublishing.org/doi/10.1098/rstl.1846.0007 |journal=Philosophical Transactions of the Royal Society of London |language=en |volume=136 |pages=103–106 |doi=10.1098/rstl.1846.0007 |s2cid=110072210 |issn=0261-0523}}</ref><ref>{{Cite journal |last=Kay |first=A. B. |date=2015 |title=The early history of the eosinophil |url=https://pubmed.ncbi.nlm.nih.gov/25544991 |journal=Clinical and Experimental Allergy |volume=45 |issue=3 |pages=575–582 |doi=10.1111/cea.12480 |issn=1365-2222 |pmid=25544991|s2cid=198242 }}</ref> and noticed the blood of a marine fish ([[Biology:Skate (fish)|skate]]) had cells that could move by themselves and remarked that "the granule-cells at first presented most remarkable changes of shape."<ref>{{Cite journal |last=Jones |first=Thomas Wharton |date=1846-12-31 |title=IV. The blood-corpuscle considered in its different phases of development in the animal series. Memoir I.—Vertebrata |url=https://royalsocietypublishing.org/doi/10.1098/rstl.1846.0005 |journal=Philosophical Transactions of the Royal Society of London |language=en |volume=136 |pages=63–87 |doi=10.1098/rstl.1846.0005 |s2cid=52938309 |issn=0261-0523}}</ref> Other scientists confirmed his findings, however, among them, German physician Johann Nathanael Lieberkühn in 1854 concluded that the movement was not for ingesting food or particles.<ref name=":5" />

Disproving Lieberkühn's conclusion, Haeckel discovered that such cells could indeed ingest particles, even experimentally introduced ones. In 1862, Haeckel injected an Indian ink (or [[Indigo|indigo]]<ref name=":7">{{Cite journal |last1=Rebuck |first1=J. W. |last2=Crowley |first2=J. H. |date=1955-03-24 |title=A method of studying leukocytic functions in vivo |url=https://pubmed.ncbi.nlm.nih.gov/13259351 |journal=Annals of the New York Academy of Sciences |volume=59 |issue=5 |pages=757–805 |doi=10.1111/j.1749-6632.1955.tb45983.x |issn=0077-8923 |pmid=13259351|bibcode=1955NYASA..59..757R |s2cid=39306211 }}</ref>) into a sea slug,''[[Biology:Tethys fimbria|Tethys]]'', and observed how the colour was taken up by the tissues. As he extracted the blood, he found that the colour particles accumulated in the cytoplasm of some blood cells.<ref name=":5" /> It was a direct evidence of phagocytosis by immune cells.<ref name=":6" /><ref name=":7" /> Haeckel reported his experiment in a monograph ''Die Radiolarien (Rhizopoda Radiaria): Eine Monographie.''<ref>{{Cite book |last=Haeckel |first=E. |date=1962 |title=Die Radiolarien (Rhizopoda radiaria): eine Monographie |url=https://books.google.com/books?id=1AqrzgEACAAJ |access-date=2023-04-07 |via=www.worldcat.org |language=en |oclc=1042894741}}</ref>

In 1869, Joseph Gibbon Richardson at the Pennsylvania Hospital observed amoeboid leukocytes from his own salivary cells, urine of an individual hospitalised for kidney and bladder problem and urine from a cystitis case. He noticed from the pus sample that one cell had moving "molecule" inside, the cell gradually enlarged and ultimately ruptured like "that of swarm of bees from a hive".<ref name=":8">{{Cite journal |last=Richardson |first=Joseph G. |date=1869-07-01 |title=Memoirs: On the Identity of the White Corpuscles of the Blood with the Salivary, Pus, and Mucous Corpusles |url=https://journals.biologists.com/jcs/article/s2-9/35/245/63578/Memoirs-On-the-Identity-of-the-White-Corpuscles-of |journal=Journal of Cell Science |language=en |volume=s2-9 |issue=35 |pages=245–250 |doi=10.1242/jcs.s2-9.35.245 |issn=1477-9137}}</ref> He hypothesised: "[It] seems not improbably that the white corpuscles, either in the capillaries or lymphatic glands, collect during their amoebaform [sic] movements, those germs of bacteria, which my own experiments indicate always exist in the blood to a greater or less amount."<ref>{{Cite web |last=Richardson |first=Joseph Gibbon |date=1869 |title=On the identity of the white corpuscles of the blood with the salivary, pus, and mucous corpuscles |url=https://collections.nlm.nih.gov/catalog/nlm:nlmuid-9601264-bk |access-date=2023-04-14 |website=Digital Collections - National Library of Medicine}}</ref><ref>{{Cite web |last=Richardson |first=Joseph Gibbons |date=1869 |title=On the identity of the white corpuscles of the blood with the salivary, pus, and mucous corpuscles |url=https://wellcomecollection.org/works/b689bgcx |access-date=2023-04-14 |website=Wellcome Collection |language=en}}</ref> Although generally overlooked in the study of phagocytosis,<ref name=":2" /> after it was originally published in the ''Pennsylvania Hospital Report'',<ref>{{Cite book |last=Richardson |first=Joseph G. |url=https://books.google.com/books?id=5aqZEAAAQBAJ&dq=%22On+the+identity+of+the+white+corpuscles+of+the+blood%22+richardson+Reports+1869&pg=PA177 |title=A Handbook of Medical Microscopy |date=2022 |publisher=Books on Demand [Verlag] |isbn=978-3-368-12882-1 |page=177 |language=en |orig-date=1871}}</ref> it was reproduced in other journals.<ref name=":8" /><ref>{{Cite journal |last=Richardson |first=Joseph |date=1870 |title=On the Identity of the White Corpuscles of the Blood, with the Salivary Pus, and Mucous Corpuscles |journal=The American Journal of Dental Science |volume=4 |issue=8 |pages=364–369 |pmc=6098400 |pmid=30752625}}</ref><ref>{{Cite journal |last=Day |first=John |date=1869 |title=On colour tests as aids to diagnosis |url=https://rest.neptune-prod.its.unimelb.edu.au/server/api/core/bitstreams/1005e06d-1a3f-55f6-8a6a-ba539f968495/content |journal=Australian Medical Journal |volume=14 |pages=333}}</ref>

=== Epithelial cells ===
In 1869, Russian physician Kranid Slavjansky published his research on injection of guinea pigs and rabbits with indigo and [[Chemistry:Cinnabar|cinnabar]] in {{Lang|de|Archiv für pathologische Anatomie und Physiologie und für klinische Medicin}} (later renamed ''Virchows Archiv'')''.''<ref name=":3">{{Cite journal |last=Slavjansky |first=Kranid |date=1869 |title=Experimentelle Beiträge zur Pneumonokoniosis-Lehre |trans-title=Experimental contributions to the theory of pneumonoconiosis |url=http://link.springer.com/10.1007/BF01986371 |journal=Archiv für Pathologische Anatomie und Physiologie und für Klinische Medicin |language=de |volume=48 |issue=2 |pages=326–332 |doi=10.1007/BF01986371 |s2cid=34022056 |issn=0945-6317}}</ref> Slavjansky found that leukocytes easily take up the indigo and cinnabar as do the cells of the respiratory tract (alveoli). He noticed that the [[Biology:Alveolar cells|alveolar cells]] behaved like the leukocytes as they became distributed in the alveoli and the bronchial mucus,<ref>{{Cite journal |last1=Matsuura |first1=Y. |last2=Chin |first2=W. |last3=Kurihara |first3=T. |last4=Yasui |first4=K. |last5=Asao |first5=M. |last6=Hayashi |first6=T. |last7=Fukushima |first7=M. |last8=Abe |first8=H. |last9=Kurata |first9=A. |date=1990 |title=[Tachycardia-induced cardiomyopathy: a case report] |url=https://pubmed.ncbi.nlm.nih.gov/2104425 |journal=Journal of Cardiology |volume=20 |issue=2 |pages=509–518 |issn=0914-5087 |pmid=2104425}}</ref> the observation of which made him to suggest that the tissue cells were the source of particle up-take in the lungs.<ref name=":2" /> He concluded:<blockquote>
{{Lang|de|Da jene Zellen zinnoberhaltig sind, so liegt es auf der Hand, sie als weisse Blutzellen anzunehmen, welche aus den Gefӓssen herauswandernd und kein freies Pigment in den Lungen-Alveolen findend, wie das der Fall in den Versuchen ist, wo man Zinnober in das Blut injicirt, nachdem man zwei Tage früher Indigo in die Lunge eingeführt hat, als zinnoberhaltige Zellen erscheinen... entweder sind es ausgewanderte weisse Blutkörperchen, welche die Schleim-metamorphose durchgemacht haben und auf diese Weise in Schleimkörperchen übergegangen sind, oder sie können von den metamorphosirten Cylinderepithelien der Bronchialschleimhaut stammen.}} [As those cells contain cinnabar, it is natural to suppose them to be white blood cells migrating out of the vessels and finding no free pigment in the pulmonary alveoli, as is the case in the experiments in which cinnabar is introduced into the blood after introducing indigo into the lungs two days before cinnabar cells appear... either they are migrated white blood cells which have undergone mucus metamorphosis and have thus become mucus corpuscles, or they can come from the metamorphosed columnar epithelium of the bronchial mucosa.]<ref name=":3" /></blockquote>
A Canadian physician William Osler at McGill College reported "On the pathology of miner's lung" in ''Canada Medical and Surgical Journal'' in 1875.<ref name=":0">{{cite journal |last1=Ambrose |first1=Charles T. |year=2006 |title=The Osler slide, a demonstration of phagocytosis from 1876: Reports of phagocytosis before Metchnikoff's 1880 paper |journal=Cellular Immunology |volume=240 |issue=1 |pages=1–4 |doi=10.1016/j.cellimm.2006.05.008 |pmid=16876776}}</ref> Osler had examined a case of black lung disease (pneumoconiosis) in two miners. From an autopsy of one who died from the disease, he found leukocytes and lung cells (alveolar cells) that contained the coal (carbon) particles.<ref name=":2">{{Cite journal |last=Rosen |first=George |date=1949 |title=Osler on Miner's Phthisis |url=https://academic.oup.com/jhmas/article-lookup/doi/10.1093/jhmas/IV.3.259 |journal=Journal of the History of Medicine and Allied Sciences |language=en |volume=IV |issue=3 |pages=259–266 |doi=10.1093/jhmas/IV.3.259 |issn=0022-5045 |jstor=24619120}}</ref> For the blood cells, he was not convinced that the coal particles were taken up by the cells; instead suggesting that "they must be regarded as the original cell elements of the alveoli", conceding that he lacked "the necessary knowledge to decide." But on the lung cells, his observation was clear, remarking:<blockquote>Inside all of these [lung cells] the carbon particles exist in extraordinary numbers, filling the cells in different degrees. Some are so densely crowded that not a trace of cell substance can be detected, more commonly a rim of protoplasm remains free, or at a spot near the circumference, the nucleus, which in these cells is almost always eccentric, is seen uncovered... One most curious specimen was observed: on an elongated piece of carbon three cells were attached, one at either end, and a third in the middle; so that the whole had a striking resemblance to a dumbbell. I could hardly credit this at first, until, by touching top-cover with a needle and causing the whole to roll over, I quite satisfied myself that the ends of the rod were completely imbedded in the corpuscles, and the middle portion entirely surrounded by another.<ref name=":1">{{Cite journal |last=Oslar |first=William |date=1875 |title=On the pathology of miner's lung |url=https://www.mcgill.ca/medicalmuseum/files/medicalmuseum/miners_lung_cmsj_1885.pdf |journal=Canada Medical and Surgical Journal |volume=4 |pages=145–169 |url-access=}}</ref></blockquote>Oslar's report continued with his experimental observation. He injected Indian ink into the axillae and lungs of kittens.<ref name=":0" /> On autopsy of a two-day-old kitten, he noticed leukocytes and large tissue cells, which showed [[Biology:Amoeboid movement|amoeboid movement]]s, containing the ink. However, he could not work out how the ink spread inside the cells, as he accidentally dropped and broke his slide. From a four-week-old kitten, he found that the ink also accumulated in almost all the blood and lung cells, and such cells were so crowded that under a microscope "hardly anything could be seen.<ref name=":1" /> He was convinced that there was a cellular process of up-taking particles ("irritating materials" as he called them<ref name=":2" />), which he considered as an "intravasation" or "ingestion", as he concluded:<blockquote>Here we have to do with an intravasation, or rather an ingestion of the coloured corpuscles within others. Many deny this, but as far as my observation goes there can be no doubt of the fact. In these corpuscles as many as six to ten were seen, in others again the outlines of the red corpuscles could not be detected, as if the cells had absorbed only the colouring matter.<ref name=":1" /></blockquote>

== Discovery of macrophage ==

=== Groundwork ===
The phagocytic property of macrophage, a specialised leukocyte, and its role in immunity was discovered by Ukrainian zoologist Élie Metchnikoff. However, he did not discover phagocytes or phagocytosis, as is often depicted in books.<ref name=":9">{{Cite journal |last1=Teti |first1=Giuseppe |last2=Biondo |first2=Carmelo |last3=Beninati |first3=Concetta |date=2016 |title=The Phagocyte, Metchnikoff, and the Foundation of Immunology |journal=Microbiology Spectrum |volume=4 |issue=2 |page=MCHD-0009-2015 |doi=10.1128/microbiolspec.MCHD-0009-2015 |issn=2165-0497 |pmid=27227301 |doi-access=free}}</ref> Metchnikoff had been working as professor of zoology and comparative anatomy at the University of Odessa, Ukraine (then Russian Empire), since 1870.<ref>{{Cite journal |last=Gordon |first=Siamon |date=2008 |title=Elie Metchnikoff: father of natural immunity |journal=European Journal of Immunology |volume=38 |issue=12 |pages=3257–3264 |doi=10.1002/eji.200838855 |issn=1521-4141 |pmid=19039772 |doi-access=free}}</ref> In 1880, he had nervous breakdown, partly due to her wife Olga Belokopytova's terminal typhoid fever, and attempted suicide by self-injecting with blood sample from blood from an individual with relapsing fever.<ref>{{Cite journal |last=Cavaillon |first=Jean-Marc |date=2011 |title=The historical milestones in the understanding of leukocyte biology initiated by Elie Metchnikoff |journal=Journal of Leukocyte Biology |volume=90 |issue=3 |pages=413–424 |doi=10.1189/jlb.0211094 |issn=1938-3673 |pmid=21628329 |doi-access=free}}</ref> By then he had keen interest in Charles Darwin's theory of natural selection, and had been investigating the origin of metazoans.<ref>{{Cite journal |last=Merien |first=Fabrice |date=2016 |title=A Journey with Elie Metchnikoff: From Innate Cell Mechanisms in Infectious Diseases to Quantum Biology |journal=Frontiers in Public Health |volume=4 |pages=125 |doi=10.3389/fpubh.2016.00125 |issn=2296-2565 |pmc=4909730 |pmid=27379227 |doi-access=free }}</ref>

Based on the knowledge of cell eating in primitive metazoans, Metchnikoff believed that the common ancestor of metazoan must be a simple cell-eating organism. His initial experimental observation in 1880 in Naples, Italy, showed that such intracellular digestion does occur in the [[Biology:Parenchyma|parenchyma]] (tissue cells) of coelenterates, and became convinced that the original metazoan must be like that.<ref name=":10">{{Cite journal |last1=Ghiselin |first1=M. T. |last2=Groeben |first2=C. |date=1997 |title=Elias Metschnikoff, Anton Dohrn, and the Metazoan common ancestor |url=https://pubmed.ncbi.nlm.nih.gov/11619470 |journal=Journal of the History of Biology |volume=30 |issue=2 |pages=211–228 |doi=10.1023/a:1004279501998 |jstor=4331432 |pmid=11619470|s2cid=2949166 }}</ref> He called this hypothetical metazoan ancestor ''parenchymella<ref name=":9" />'' (later commonly known as ''phagocytella'';<ref name=":11">{{Cite journal |last1=Chernyak |first1=Leon |last2=Tauber |first2=Alfred I. |date=1988 |title=The birth of immunology: Metchnikoff, the embryologist |url=https://linkinghub.elsevier.com/retrieve/pii/0008874988900901 |journal=Cellular Immunology |language=en |volume=117 |issue=1 |pages=218–233 |doi=10.1016/0008-8749(88)90090-1|pmid=3052859 }}</ref> the term parechymella adopted for the name of the larvae of demosponges.<ref>{{Cite journal |last=Vaughan |first=R. B. |date=1965 |title=The romantic rationalist: A study of Elie Metchnikoff |journal=Medical History |volume=9 |issue=3 |pages=201–215 |doi=10.1017/s0025727300030702 |issn=0025-7273 |pmc=1033501 |pmid=14321564}}</ref><ref>{{Cite journal |last1=Renard |first1=Emmanuelle |last2=Vacelet |first2=Jean |last3=Gazave |first3=Eve |last4=Lapébie |first4=Pascal |last5=Borchiellini |first5=Carole |last6=Ereskovsky |first6=Alexander V. |date=2009 |title=Origin of the neuro-sensory system: new and expected insights from sponges |journal=Integrative Zoology |volume=4 |issue=3 |pages=294–308 |doi=10.1111/j.1749-4877.2009.00167.x |issn=1749-4877 |pmid=21392302|doi-access=free }}</ref>) This was a direct contradiction to the hypothesis of [[Biography:Ernst Haeckel|Ernst Haeckel]], a German zoologist and staunch supporter of Darwin's theory. In 1872, Haeckel had formulated a theory (as part of his evolutionary theory called biogenetic law) that a metazoan ancestor must be like a gastrula, an embryonic stage undergoing [[Philosophy:Invagination|invagination]] as seen in chordates.<ref>{{Cite journal |last1=Levit |first1=Georgy S. |last2=Hoßfeld |first2=Uwe |last3=Naumann |first3=Benjamin |last4=Lukas |first4=Paul |last5=Olsson |first5=Lennart |date=2022 |title=The biogenetic law and the Gastraea theory: From Ernst Haeckel's discoveries to contemporary views |url=https://pubmed.ncbi.nlm.nih.gov/33724681 |journal=Journal of Experimental Zoology. Part B, Molecular and Developmental Evolution |volume=338 |issue=1–2 |pages=13–27 |doi=10.1002/jez.b.23039 |issn=1552-5015 |pmid=33724681|s2cid=232242294 }}</ref> He named the hypothetical ancestor ''gastrea.<ref name=":10" />''

=== Experimental discovery ===
To strengthen his parenchymella theory, Metchnikoff thought about several ways to look for cell eating as a fundamental process in metazoans.<ref name=":11" /> In the summer of 1880, he resigned from the University of Odessa and moved to Messina, a seashore city in Sicily, where he could conduct a private research. His initial study on sponges indicated that the [[Biology:Mesoderm|mesoderm]]al and [[Biology:Endoderm|endoderm]]al (body tissue wall) cells performed amoeboid movements and cell eating. His earlier experiments on [[Biology:Planarian|planarian]] worms already showed that the endoderm is formed by migrating cells, and not by invagination.<ref name=":14">{{Cite journal |last1=Korzh |first1=V. |last2=Bregestovski |first2=P. |date=2016 |title=Elie Metchnikoff: Father of phagocytosis theory and pioneer of experiments in vivo |url=http://link.springer.com/10.3103/S0095452716020080 |journal=Cytology and Genetics |language=en |volume=50 |issue=2 |pages=143–150 |doi=10.3103/S0095452716020080 |pmid=27281928 |s2cid=255429705 |issn=0095-4527}}</ref> His critical study came from the larvae ([[Biology:Bipinnaria|bipinnaria]]) of a starfish, ''Astropecten pentacanthus'' (later reclassified as ''Astropecten irregularis'').<ref>{{Cite journal |last=Cohen |first=Schlomo |date=2008 |title=New interpretation of vasculitis in the light of evolution |url=https://pubmed.ncbi.nlm.nih.gov/18552578 |journal=The American Journal of the Medical Sciences |volume=335 |issue=6 |pages=469–476 |doi=10.1097/MAJ.0b013e318173e1b0 |issn=0002-9629 |pmid=18552578}}</ref>

Metchnikoff observed that the body covering of the transparent starfish consisted of the outer ([[Biology:Ectoderm|ectoderm]]) and internal (endoderm) layers, and that the space in between the layers are filled with moving endodermal cells. When he injected [[Chemistry:Carmine|carmine]] stain (a red dye) into the starfish, he found that the stain was taken up (eaten) by the amoeboid cells as they turned red in colour.<ref name=":14" /> He remarked: "I found it an easy matter to demonstrate that these elements seized foreign bodies of very varied nature by means of their living processes, and certain of these bodies underwent a true digestion within the amoeboid cells."<ref name=":12" /> Then, he conceived a novel idea that if the cells could eat external particles, they must be responsible for eating harmful materials and pathogens like bacteria to protect the body – the key process to immunity.<ref name=":14" />

It was one afternoon in December 1880, when he stayed home alone while his family went to a circus show that he momentarily realised that his idea could be put to test by piercing live starfish larvae. He collected fresh specimens from the seashore and a few rose thorns on the way home.<ref>{{Cite journal |last=Trimarchi |first=F. |date=1993 |title=Czarist police, roses, seashore, performing apes and phagocytosis |journal=Journal of the Royal Society of Medicine |volume=86 |issue=4 |pages=225 |doi=10.1177/014107689308600414 |issn=0141-0768 |pmc=1293955 |pmid=8505733}}</ref> He discovered what he hypothesised, that the amoeboid cell gathers round the rose thorn as if to eat when it pierced through the skin, and predicted that the same would be true in humans as a form of body defence.<ref name=":12" /> Recapitulating the experiment, he said:<blockquote>I hypothesized that if my presumption was correct, a thorn introduced into the body of a starfish larva, devoid of blood vessels and nervous system, would have to be rapidly encircled by the motile cells, similarly to what happens to a human finger with a splinter. No sooner said than done. In the shrubbery of our home, the same shrubbery where we had just a few days before assembled a 'Christmas tree' for the children on a mandarin bush, I picked up some rose thorns to introduce them right away under the skin of the superb starfish larva, as transparent as water. I was so excited I couldn't fall asleep all night in trepidation of the result of my experiment, and the next morning, at a very early hour, I observed with immense joy that the experiment was a perfect success! This experiment formed the basis for the theory of phagocytosis, to whose elaboration I devoted the next 25 years of my life.

Thus, it was in Messina that the turning point in my scientific life took place.<ref name=":13" /></blockquote>

== References ==
{{Reflist}}

[[Category:Immunology]]
[[Category:Cellular processes]]

{{Sourceattribution|History of phagocytosis}}