Blood donation is the process in which a donor voluntarily donates blood for transfusion to a person who requires blood or for a process known as fractionation (in which the different components of the blood are separated). Whole blood or individual blood components are donated (known as apheresis).
Blood banks all around the world usually take part in collection of blood, separation of the various blood components.
There are various techniques to collect blood; blood can be collected either manually or by using automated machines.

The different types of blood donation
There are different types of blood donation; namely
1. Whole blood: this is the most common type of blood donation; about a pint of whole blood is collected from the donor and then the different components of blood namely the red blood cells, white blood cells and platelets of blood are separated
2. Platelet donation: this type of blood donation is also known as apheresis. In this process the platelets are collected along with some amount of plasma and returned the rest of the collected blood to the donor
3. Plasma donation: in this process the plasma is collected alone or without the platelets through the apheresis process
4. Double red cell donation: in this process only the red blood cells (RBCs) are collected using the process of apheresis
Other than the classification based on the nature of the collected blood or blood component; blood donation is also classified based on the recipient of the blood, as
1. Allogenic or homologous transfusion in which the blood donated is stored at the blood bank for transfusion to an unknown person who requires blood transfusion
2. Directed donation (transfusion): in this type of blood transfusion, a donor donates blood to a specific person especially a family member friend or acquaintance
3. Replacement donation: in this type a person requiring blood transfusion gets blood from the blood bank and that transfused blood is replaced by donation of blood to the blood bank by a friend or family member of the person. In this way a consistent source of blood supply is always maintained
4. Autotransfusion: in this way persons who may require blood transfusion (as those undergoing major surgery) in future can store their own blood for future use

Screening tests
Several screening tests are done in order to ensure both the recipient’s safety as well as donor’s safety.
1. For recipient’s safety: certain health conditions in the donor may make the donated blood unsafe for recipients. Hence the donors are screened for these conditions prior to blood donation. These are HIV, malaria, viral hepatitis etc. In some countries there are some controversial restrictions as who can donate blood; these are restriction of blood donation by MSM (men who have sex with men) for increased risk of HIV in these people.
2. For donor’s safety: donors are to undergo medical examination and are enquired about their past medical history to ensure that blood donation is not going put their health at risk. Hemoglobin levels and other blood tests (tests to estimate the hematocrit levels) are undertaken along with physical examination of pulse, measurement of blood pressure (people with high or low blood pressure are usually deferred) and body temperature. Elderly people and pregnant women are usually deferred.  Donor should undertake some preparations before donating blood.

Some other tests are undertaken on the collected blood for the purpose of donor’s safety and to ascertain which of the collected blood is suitable for which recipient. The collected blood is tested for blood grouping (A, B, AB and O), donor’s Rh typing and for presence of other no so common antigens. Group O blood is considered to be the universal donor for red cell transfusion and group AB is considered as universal donor for plasma transfusion.
Some other additional screening tests are also undertaken depending local area like West nile virus, cytomegalovirus, etc.