The strength with which an antibody binds to an antigen is known as its affinity. Depending on the practical use which is required of the antibody, it is possible that strong binding of antibody to an antigen may be, under different circumstances, a good or bad thing. Under normal conditions the antibody-antigen interaction occurs so that the body may eliminate potentially harmful substances. Secondary reactions take place so that the complexed antigen may be recognized and eliminated. However, as will be seen later, in clinical practice it may be desirable to introduce an antibody into a patient in order to find a particular type of cell or tissue. Under these circumstances clearly it would be very undesirable if the binding characteristics of the monoclonal antibody to the normal tissue were such that complement fixation occurred and the antibody caused the death of a normal tissue. In other circumstances, monoclonal antibodies may be introduced into patients in order to eliminate drugs or poisons. Here it is clearly essential that they should retain the ability to help in excretion of the unwanted antigens.
