Why donate blood? There is tremendous demand for blood in hospitals. Many patients die because they are not able to cope with the loss of blood. The blood donated is used to: Replace blood lost during injury as in accidents. Replace blood loss during major surgeries. Help patients with blood disorders like haemophilia survive. Help burnt patients receive plasma that may be critical for their survival. Raise haemoglobin levels (through transfusions) in patients with chronic ailments like kidney diseases, cancer and anaemia.

A blood donation is when a healthy person voluntarily has blood drawn. The blood is used for transfusions or made into medications by a process called fractionation. In the developed world, most blood donors are unpaid volunteers who give blood for a community supply. In poorer countries, established supplies are limited and donors usually give blood when family or friends need a transfusion. Many donors donate as an act of charity, but some are paid and in some cases there are incentives other than money such as paid time off from work. A donor can also have blood drawn for their own future use.

Blood banks sometimes use a modified bus or similar large vehicle to provide mobile facilities for donation. Blood donations are divided into groups based on who will receive the collected blood. An allergenic (also called homologous) donation is when a donor gives blood for storage at a blood bank for transfusion to an unknown recipient. A directed donation is when a person, often a family member, donates blood for transfusion to a specific individual. Directed donations are relatively rare. A replacement donor donation is a hybrid of the two and is common in developing countries such as Ghana. In this case, a friend or family member of the recipient donates blood to replace the stored blood used in a transfusion, ensuring a consistent supply. When a person has blood stored that will be transfused back to the donor at a later date, usually after surgery, that is called an antilogous donation. Blood that is used to make medications can be made from allergenic donations or from donations exclusively used for manufacturing.

Donors are screened for health risks that might make the donation unsafe for the recipient. Some of these restrictions are controversial, such as restricting blood donations from men who have sex with men for HIV risk. Autologous donors are not always screened for recipient safety problems since the donor is the only person who will receive the blood. Donors are also asked about medications such as dutasteride since they can be dangerous to a pregnant woman receiving the blood. Donors are examined for signs and symptoms of diseases that can be transmitted in a blood transfusion, such as HIV, malaria, and viral hepatitis. Screening may extend to questions about risk factors for various diseases, such as travel to countries at risk for malaria or variant Creutzfeldt - Jakob disease (vCJD). These questions vary from country to country. For example, while Québec may defer donors who lived in the United Kingdom for risk of vCJD, donors in the United Kingdom are only restricted for vCJD risk if they have had a blood transfusion in the United Kingdom.

The donor's blood type must be determined if the blood will be used for transfusions. The collecting agency usually identifies whether the blood is type A, B, AB, or O and the donor's Rh (D) type and will screen for antibodies to less common antigens. More testing, including a crossmatch, is usually done before a transfusion. Group O is often cited as the "universal donor" but this only refers to red cell transfusions. For plasma transfusions the system is reversed and AB is the universal donor type. Most blood is tested for diseases, including some STDs. The tests used are high-sensitivity screening tests and no actual diagnosis is made. Some of the test results are later found to be false positives using more specific testing. False negatives are rare, but donors are discouraged from using blood donation for the purpose of anonymous STD screening because a false negative could mean a contaminated unit. The blood is usually discarded if these tests are positive, but there are some exceptions, such as autologous donations. The donor is generally notified of the test result.