Blood typing procedure-agglutination

In the blood typing procedure what causes agglutination of red blood cells? Use the words "antigen" and "antibody"

I shall explain this using the example of antigen A

Antigen A is present on the surface of the RBC. In the testing procedure anti-A serum contains antibodies specific for antigen A, that is, anti-A-antibody. The anti-A-antibody has multiple binding sites for antigen A. When the test drop of blood is mixed with anti-A serum, these anti-A-antibodies bind to the antigens A present on the RBC surface. Because of their multiple binding sites one antibody molecule can bind to a antigen on cell-1 and also to antigen on cell2 simultaneously. Thus it brings the two cells close to one another. Thousands of such antibody molecules thus can bring together lots of cells in clumps and cause agglutination.

scroll down the page on this link to find the diagram that explains this phenomenon
http://waynesword.palomar.edu/aniblood.h…

According to the AB0 blood group system there are four different kinds of blood groups: A, B, AB or 0 (null).
Blood group A
If you belong to the blood group A, you have A antigens on the surface of your red blood cells and B antibodies in your blood plasma.
Blood group B
If you belong to the blood group B, you have B antigens on the surface of your red blood cells and A antibodies in your blood plasma.
Blood group AB
If you belong to the blood group AB, you have both A and B antigens on the surface of your red blood cells and no A or B antibodies at all in your blood plasma.
Blood group 0
If you belong to the blood group 0 (null), you have neither A or B antigens on the surface of your red blood cells but you have both A and B antibodies in your blood plasma.
In cross-matching, agglutination occurring when donor red blood cells and recipient's serum or plasma are incubated together indicates that the donor blood is incompatible for that particular recipient.