What are antibodies? How do they help in detecting the blood group type?

The human immune system has various ways of responding to an infection caused by bacteria or viruses. Our bodies produce proteins (antibodies) that are highly specific for the infectious agent as a part of our "humoral" immune response. The antibodies help stop the infection from spreading further and help to eliminate the bacteria or virus from the body.Antibodies are also used to help our bodies find and destroy "foreign" cells such as tumors.
Because antibodies bind tightly to only one type of structure on the surface of cells (antigen), they can also be useful for identifying different types of blood cells. It is important to correctly identify blood cells in our bodies if we ever need to receive blood from someone else because we are sick (transfusion). Our blood type is determined based on the presence or absence of two proteins on the surface of our red blood cells (Type A and Type B). There are four possible combinations of blood types namely: Type A, Type B, Type AB, and Type O (contains neither A nor B proteins). This is referred to as the ABO blood type. In addition, red blood cells have a Rhesus factor or Rh, which is either present or absent. If the Rh factor is present, the cells are referred to as Rh positive. Including both the ABO and Rh systems for blood typing, there are a total of 8 possible blood types.