THE two beneficiaries of this year's Operation Santa Claus - the Hong Kong Marrow Match Foundation (HKMMF) and the Paediatric Bone Marrow Transplant Unit at the Prince of Wales Hospital - are both involved in bone marrow transplantation (BMT) using marrow from unrelated donors. This procedure has been technically possible for more than 20 years, but only in the past five years or so has it become recognised as a realistic form of treatment for certain blood diseases. The principle is remarkably simple. All the body's blood cells are produced from so-called ''stem cells'' found in the bone marrow inside some of the larger bones. If a patient has a serious blood disease such as leukaemia, in which the marrow produces too many white blood cells, or aplastic anaemia, in which too few cells are produced, the patient's defective marrow is intentionally destroyed by drugs or radiotherapy and is replaced by marrow taken from a healthy volunteer. The principle is simple, but the procedure requires critical matching of the donor and recipient. This matching makes use of factors called ''tissue types'' or ''human leucocyte antigens'' (usually written as ''HLA antigens''). These may be thought of as a very complex form of blood grouping. Unlike the four familiar blood types, O, A, B and AB, used for blood transfusion matching, the tissue types used to match BMT donors and recipients run into thousands of possible types. Everyone has their own personal set of tissue types inherited from their parents. These types consist of three major groups known as HLA-A, HLA-B, and HLA-DR. For each of these major groups, we inherit two tissue types, one from each parent. We therefore have two HLA-A antigens, two HLA-B antigens and two HLA-DR antigens. There are known to exist over 40 different HLA-A types, over 70 HLA-B types and over 60 HLA-DR types. The number of possible combinations of two HLA-A antigens, two HLA-B antigens and two HLA-DR antigens from the ones which are currently known about runs into hundreds of thousands. For a successful BMT, the HLA antigens of the patient must exactly match those of the donor. Family members have the greatest chance of having matched tissue types. Because brothers and sisters of the same family inherit their tissue types from the same parents, the chances of a patient finding a matched donor within his or her family are much higher than within the population as a whole. For each sibling of the patient there is a one in four chance that the sibling's tissue types match those of the patient. Patients with large families naturally have a higher chance of finding a matched donor than those with smaller families. In Hong Kong, we find that about 40 per cent of patients have a fully matched sibling. The patients who are most disadvantaged in terms of finding a matched sibling are clearly those from small families, and particularly the first-born, when the disease is diagnosed within the first year or so of life. What can we do to help the 60 per cent of patients who do not have a matched sibling available? The answer lies with the many unrelated bone marrow donor registries that have been formed around the world in the past few years, such as the one set up two years ago by the HKMMF. These registries rely upon the generosity of volunteers who come forward anonymously to have their tissue types recorded in a computer file to be compared with the tissue types of patients who need a BMT but do not have a suitable related donor. Plans for a Hong Kong registry began more than three years ago, soon after Hong Kong's first BMT was performed using a related donor in early 1990. At that time, unrelated transplantation was well-established overseas and doctors here acknowledged that it would be only a matter of time before the treatment became available in Hong Kong. However, funds in Hong Kong were limited. The preliminary blood test for a single donor costs $500. If a donor is found to be a preliminary match, a second blood test is necessary which costs $800. IT soon became clear that an unrelated donor registry would be a luxury for which we could not expect any ''official'' funding. Plans were therefore made to set up the HKMMF as a charity to fund and administer an unrelated donor registry for Hong Kong. The foundation eventually received its status as a registered charity in November 1991, just at the time that two-year-old Canadian-Chinese Gordon Wu was brought to Hong Kong by his parents in search of an unrelated BMT donor. The first recruitment drive was held during one weekend in November 1991, during which over 9,000 donors came forward to join the registry. Today the registry has over 16,000 donors on its files and is the largest registry of Chinese donors in the world. In the past two years, the HKMMF has searched for donors for 138 patients in Hong Kong and 129 overseas. Sixty-two of the Hong Kong patients and 16 of the overseas patients have found fully matched donors. Of these, 18 patients in Hong Kong and six overseas have received transplants. The question is frequently asked why so few of the patients with matched donors have actually received a transplant. In most cases the answer is that the patients are currently in remission and will not be considered for transplant until the disease relapses. Some are already on the waiting list for a transplant and will be transplanted within the next few months. But in some cases, the donor is found too late in the course of the patient's disease and the patient is too ill to benefit from a transplant. The HKMMF is proud of its achievements in the past two years. However, we cannot afford to be complacent. Although the foundation is able to find donors for 45 per cent of local patients for whom it performs searches, there are still 55 per cent who cannot find a donor. The way to help these patients is to recruit more donors on to the donor registry. We can calculate that by boosting the size of the registry to 25,000 we can achieve a 60 per cent success rate. While this may not seem an impressive improvement over 45 per cent, it does mean that perhaps 15 extra patients a year could find a donor and have a reasonable chance of conquering their disease. For these 15 patients, the difference between 45 per centand 60 per cent is literally the difference between life and death. As the registry grows beyond 25,000 donors, the success rate will continue to increase. However, we have to face the fact that we will never achieve a 100 per cent success rate. In fact, even if we were to tissue type everyone in the world there would still be some patients who could not find a matched donor. Many factors play a part in determining the chance of finding a matched donor, but the most important are genetic factors, particularly those related to the racial origin of the patient. This is why the Hong Kong registry concentrates its efforts on recruiting Chinese donors. Our emphasis on Chinese donors does not carry any racist or any other implications. It simply reflects the biological reality that the best place to find a donor is among people of the same racial group as the patient. Generally speaking, Chinese patients need Chinese donors, Japanese patients need Japanese donors, northern European patients need northern European donors and so on. This is all a matter of simple genetics. Some differences between races are clearly visible, such as hair colour, skin colour, facial structure, etc. These differences are determined by genes which have been handed down from one generation to another over thousands of years of evolution. As different races have evolved separately they have tended to maintain their own particular set of genes. Among these are the genes which determine tissue types. Chinese people therefore have a very different set of tissue types to Caucasians. They are more similar to Japanese and Koreans, but even so, they have very distinct tissue types. Even Chinese from different parts of China have different tissue types -northern Chinese are quite distinct from southern Chinese. For these reasons the HKMMF made the conscious decision to be a ''Chinese'' registry and is now recognised throughout the world as the place to search for donors for Chinese patients. For those who are committed to becoming bone marrow donors but are not Chinese we would always recommend that they join one of the registries overseas. It is not because we do not want them as donors. The fact is that if they join the Hong Kong registry they will probably never be called as a marrow donor because they will not match the patients who are searching for donors. They are more likely to match patients of similar origin to themselves and would be more valuable in a registry that routinely searches for such patients. An exception to this is for donors who have one Chinese parent and one parent of another racial origin. Because of the way individual races have different sets of tissue types, patients whose parents are of different racial backgrounds have particular difficulty in finding unrelated donors. Therefore, donors with one Chinese parent and one non-Chinese parent are particularly welcome on the registry. There are various ways for a prospective donor to join the registry. The HKMMF collaborates closely with the Red Cross Blood Transfusion Service. Prospective donors can visit the Red Cross Blood Donor Centre at Shop 330 Omni-Hong Kong Hotel Shopping Centre next to Ocean Terminal in Tsim Sha Tsui any Wednesday between noon and 6 pm. The donor will be asked to complete a brief medical questionnaire and to sign a consent form before 10 cc of blood (equivalent to two teaspoons) is collected. From time to time, the foundation also arranges mass recruitment campaigns around the territory. These are advertised in the press and tend to target donors living in a particular area. A third method is for companies, organisations and community groups to conduct their own ''corporate drives''. If such a group has 50 or so members who wish to become donors, the foundation will be happy to visit the group during working hours and collect blood from everyone who wishes to join the registry. Any group wishing to organise a corporate drive is invited to call the foundation's donor co-ordinator on 819-0766. Once a donor is found to be a preliminary match with a patient, the donor will be asked to have another blood test for the second level of tissue types (HLA-DR). IF these tests also match the patient, the donor will then be asked to have a full medical examination and additional blood tests. The nature of the bone marrow donation process will be explained in detail and the donor will be asked to consider carefullywhether he or she wishes to proceed with the donation. If the donor wishes to proceed, the donor will be asked to sign a consent form. A few days before the planned transplant, the patient will be given drugs or radiotherapy to completely remove his own marrow. The patient is then totally reliant on the donor. Although the donor could still legally withdraw at this stage, it would be morally unacceptable, because the patient would be left without an immune system and unable to produce any more blood cells, good or bad. The donor will go into hospital the night before the transplant. The following morning the donor receives a general anaesthetic in preparation for the marrow removal. While the donor is under anaesthetic in the operating room, a needle is inserted into the back of the pelvic bone and a small amount of marrow is sucked out. The needle is then removed and reinserted a short distance away to enable more marrow to be removed. This procedure is repeated until the required amount of marrow is obtained. This amount depends on the age and size of the patient - a small baby will need much less then a large adult. After the donation process, the donor returns to the ward and will usually spend that night resting in hospital. Meanwhile the donated marrow will be given to the patient through a vein in the arm in the same way as for a blood transfusion. The donor will leave hospital the following day and will be advised to take it easy for a day or so. By then, the donor will already be replenishing his own marrow and within three weeks will have completely replenished what was removed. The donor may feel some tenderness around the donation sites for a few days, but will usually be fully recovered within a week. Anyone can be a bone marrow donor provided they are in good health and between the ages of 18 and 55. The most important requirement is commitment. The HKMMF recognises and respects the right of volunteers to change their mind at any time about being a donor. We do, however, respectfully ask that people think very carefully, and if necessary discuss the matter with their family, before they sign upas a donor. Firstly, at $500 for the initial blood test, it does not make economic sense to recruit people who will probably decline if asked to donate in the future. Secondly, and perhaps more importantly, it gives false hope to the desperate patients when people on the registry are found to be a match and subsequently decline further blood tests. Patients and their families naturally ask their doctors how many possible donors they have. Whatever number this is, it becomes the patient's lifeline. It is unfair on the patient to shorten this lifeline by telling them that some of the matched donors have refused to come back for follow-up tests.