I think clumping is caused by the mixture of antigen and the corresponding antibody. Agglutination( clumping) is the process that occurs when an antigen is mixed with its corresponding antibody. This is commonly used in blood grouping. An example is the clumping of red blood cells in the presence of an antibody or complement. In blood transfusion Clumping indicates that the blood has reacted with a certain antibody and is therefore not compatible with blood containing that kind of antibody.
