Usually none. Tranfusion of packed red blood cells will perhaps 1 time in 25 result in some mild allergy symptoms (sneeze, itch, etc.) if you're allergic to something in the blood; 1 in 100 you get a mild fever because you're allergic to donor transplant antigens. Most common serious problem is lung damage. A few others exist. Risk of any infection is extremely low.
Depends on need. While blood transfusions are generally safe this treatment does have side effects and should be used judiciously. However, far more important issue is the need for blood transfusion, given that half the transfused blood goes to people who are dead in six months. The need for transfusion generally is an indication of serious health issues.
Many. "mild" side effects can include fever during the transfusion. Most other side effects I would not consider mild and can include both hemolytic and non-hemolytic transfusion reactions, viral or bacterial infections, transfusion related lung injury, temporary suppression of the immune system among others.
Several possibly. Reactions to blood transfusions are very rare now with the techniques that we have to "crossmatch" blood. The most common reaction is have a rise in temperature. Next would be an allergic reaction. Most of these are not severe. The worst reaction is called a hemolytic reaction. These occur when there are important incompatibilities between your blood type and the type you have been given.
Rigors, fever. Rigors, occasionally fevers. Uncommon might be severe reaction. Anaphylaxis. Hperkalemia.
Immunologic or not. Causes for transfusion reactions may be immune related or not, also they can be acute (less than 24 hours) or delayed. These are some: hemolytic; febrile, non-hemolytic; bacterial contamination (septic shock); anaphylaxis; transfusion-related acute lung injury (trali); transfusion-associated circulatory overload (taco); graft vs. Host disease (ta-gvhd); iron overload; infections.
Common are trivial. We pathologists run the blood bank and are always enouraging prudent and sparing use of our product. There's a few percent chance you'll get the sneezies and/or the itchies, a bit less that you'll get a mild fever. The very serious side effects are much less common. The one that's most worrisome is lung damage from donor antibodies -- less now that we use donors less likely to have them.