I think Steven Silz-Carson has given a fine answer but I am a very visual person, and perhaps the questioner may see it more easily with a picture. read more
When you say "why do antigens clump" I immediately see IgG, one of the most common antibodies, which is a rather large protein. If you stood up and extended your arms to a V shape, your hands would be the binding sites. The sites are very specific and the two sites are identical within a given IgG molecule. read more
The most effective way for antibodies to destroy antigens is by activating complement proteins to help attack the antigens. These proteins can do a variety of things. They can rupture the invading cells, promote clumping of the cells, or weaken the foreign antigens. These proteins react in the way that is most efficient for dealing with the antigen that is recognized. read more
IgG– These are extremely important antibodies which stimulate phagocytes. They are the ones that a mother passes on internally to a child for immunity. IgM – This is the largest antibody. It also helps in the activation of B-cells. Antigen-Antibody reaction. An antigen and antibody reaction works like a lock and key mechanism. read more