Platelets are tiny fragments of bone marrow cells which circulate in the blood, clumping together to form clots which prevent bleeding following injury.
Platelet transfusions are often a vital part of treatment for many premature babies who can be highly susceptible to brain haemorrhages. Transfusions of just two or three teaspoons can cut their risk of dying.
Platelets are also given to patients whose bone marrow is not working properly.
They include unborn children who need transfusions in the womb and patients with cancer or leukaemia undergoing chemotherapy.
Demand for platelets is rising and is up by 3% so far this year.
WHO CAN DONATE?
Must have given blood without any problems
Have blood groups O, A or B
Must not be taking an anti-inflammatory painkiller
In order to donate two or three doses of platelets, you need to have a higher than average platelet count
Supplies are particularly needed over the Christmas and New Year period, when donations are down and as platelets only have a five day shelf-life, the National Blood Service (NBS) is desperate for more donors.
Platelets can be donated more frequently than whole blood because no red cells are taken, so iron levels are not affected.
But not everybody who is a blood donor can give platelets.
Men often make more suitable donors as they have a larger volume of blood and generally bigger veins - of current blood donors about 50% of males would be suitable to donate platelets and about 20-25% of females.
During the process donors are linked to a machine which takes their blood and separates the platelets. These are collected while the rest of the blood is returned to the donor.
One donation collected by this method can provide two or three therapeutic platelet doses.
Platelet donors are encouraged to donate regularly, on average ten times a year.
They must be aged between 18 and 60, and have given at least one donation of whole blood.
Dr Sheila MacLennan, Clinical Director, NHS Blood and Transplant, said that over the Christmas and New Year period stocks are desperately needed, particularly of platelets, which have a short life.
"Platelets are a very small blood cell but are a very vital blood cell as they are used for very sick patients. They play a vital part in helping blood to clot.
"The platelets only last five day. Over bank holidays we do have extra sessions, but they do take out the production capacity, and we don't run all the sessions we would.
"So we really need to remind donors to continue to replenish our stock because the patients don't stop needing them because it is a bank holiday."
Read a personal story about platelet giving here http://news.bbc.co.uk/1/hi/health/7758706.stm
and another here http://images.google.co.uk/imgres?imgurl=http://www.bonemarrowdonorsupport.com/journal/platelets.jpg&imgrefurl=http://www.bonemarrowdonorsupport.com/journal/marianne_platelets.htm&usg=__7Xh_XBS57uRNpvR2pOYsRXQO9iY=&h=1188&w=900&sz=204&hl=en&start=19&um=1&tbnid=X7_x6HgwbAe7YM:&tbnh=150&tbnw=114&prev=/images%3Fq%3Dplatelets%26um%3D1%26hl%3Den%26sa%3DN
Or talk to Anthony Booth, Bridgnorth ASM, who has given blood for many years and platelets for several years.