Adrianus Pijper, pathologist and microbiologist, younger brother of Cornelis Pijper*, was the son of Fredrik Pijper, professor of church history at the University of Leiden, and his wife Maria Petronella, born van Wijk. He received his secondary education at the Leiden Gymnasium (1898-1904) and then studied medicine at the University of Leiden. While a senior student during 1910-1912 he held assistantships in physiology, histology, anatomy, pathology and bacteriology. He qualified as Doctor of Medicine (MD) in 1912 with a thesis titled Het Coli-Vaccin (The Coli vaccine). After qualifying he received a travelling scholarship from the university and used it to study at the School of Tropical Medicine and Hygiene in Hamburg, Germany, where he obtained the Diploma in Tropical Medicine in 1912. That same year, like his elder brother, he qualified as a Licentiate in Medicine and Surgery of the Society of Apothecaries (London).
Pijper came to South Africa in 1913 and, again like his elder brother, settled in Bethal, Mpumalanga, to practise medicine. He also continued his research in bacteriology and pathology by equipping a private laboratory with the limited facilities available to him in a small town. In 1920 he moved to Pretoria, where he became a successful consulting pathologist with his own private laboratory. He served as pathologist to the Pretoria General Hospital (1920-1951), the Pretoria Mental Hospital (1922-1951), and the Pretoria Municipal Health Department; as medical officer in charge of the municipal venereal clinics of Pretoria for some years from 1922; as medical assessor for vital statistics at the South African Census Office; as a member of the Westfort Leprosy Committee (1925-1951) and Leprosy Advisory Board of South Africa (1924-1939); and as the first professor of pathology and director of the Institute of Pathology at the University of Pretoria from 1946 to his retirement in 1951. After his retirement he continued his research with financial and other assistance from the University of Pretoria and the Council for Scientific and Industrial Research, until shortly before his death.
In his research Pijper focussed on an extensive range of medical and scientific problems that arose in his work as a consulting pathologist. He was an original thinker who insisted on the highest possible integrity in scientific research, was very skilled in designing his own equipment, and published his findings in over 150 scientific articles. His research dealt mainly with the following topics: (1) The recognition of tick-bite fever (with Dr J.M. Troup, The Lancet, 1931), a disease closely allied to South African typhus and caused by a parasitic micro-organism (Rickettsia rickettsi var. pijperi) that Pijper identified. His early papers on this topic included 'The aetiology of tick-bite fever' (Journal of Tropical Medicine and Hygiene, 1930) and papers in the British Journal of Experimental Pathology (1930, 1931). (2) The distribution of the different blood groups among the South African population, reported in the Proceedings of the Royal Academy of Amsterdam (1929, for the South African Dutch), the Transactions of the Royal Society of South Africa (1930, for the Bantu), the Journal of the Medical Association of South Africa (1932, for the San), and the South African Medical Journal (1935, for the Khoi). (3) The diffraction method of determining the size of microscopic particles such as red blood cells, and the construction of an instrument which applied this method and proved useful in the diagnosis of rare blood diseases. His experiments, which started in 1918, were recognised by the South African Institute for Medical Research as original work of high quality and rewarded with a donation of 100 guineas (105 pounds sterling) in 1920. Soon apparatus constructed along the lines indicated by him were in use in hospitals and laboratories all over the world. This work was published in various articles in the Medical Journal of South Africa (1918, 1919), the South African Medical Record (1919, 1923, 1925), the South African Journal of Science (1919), and the British Medical Journal (1929). (4) The development of a satisfactory method for diagnosing typhoid and related fevers by means of complement-fixation, first reported in the Journal of the Medical Association of South Africa (1927). His method was subsequently simplified and brought into line with those of other researchers. (5) The introduction in South Africa of fever therapy for paresis (a psychosis caused by syphilitic infection of the brain), described in the South African Medical Record (1926) and later papers. (6) The first detection of the fungal disease sporotrichosis in South Africa in his laboratory in 1925 (by his colleague Dr B.D. Pullinger) reported in The Lancet (1927) and the study of the fungus that caused it (e.g., Journal of the Medical Association of South Africa, 1931); also the study of some other fungal diseases and the discovery and description of several new pathogenic fungi. Some of his early work in this area was reported in five papers on 'Unusual infections' (Medical Journal of South Africa, 1917) dealing with Thrush, Moniliasis, Nocardiasis, Nocardia infection of the bladder, and Coccidiosis. (7) The extension of dark-field methods in microscopy by replacing artificial light sources by direct sunlight, which enabled him to observe the delicate flagella of certain bacteria in the living state. His meticulous observations convinced him that bacteria were not moved by their flagella, but by undulatory movement of the bacterial cell itself, a view that led to much controversy.
Pijper served as a member of the South African Medical Council from 1941 to 1948. He became a member of the South African Association for the Advancement of Science in 1919 and served as its president in 1943. In 1920 he became a member of the South African Biological Society and later served as its president for 1925 and 1930.
The honours and awards that Pijper received included honorary doctoral degrees from the University of Pretoria (1943) and the University of Cape Town (1953), the South Africa Medal (gold) of the South African Association for the Advancement of Science (1933), the Senior Captain Scott Memorial Medal of the South African Biological Society (1930), the Hamilton Medal of the South African Medical Association (1924), the Silver Medal of the South African Medical Association (1958), and the King George V Jubilee Medal (1935). He was elected an Honorary Fellow of the Royal Microscopical Society (1934), and a Fellow of the Royal Society of South Africa (1934).
In 1913, in Leiden, Pijper married Nelly Margaretha Marie Kluyver, with whom he had two sons and a daughter.