Where do the antigen and antibody occur in the blood?

Antigen is a foreign substance to the body that illicit an immune response. Antibodies are proteins produced by lymphocytes as a result of stimulation of immune system. Antigens are present on the surface of the red blood cells. Antibodies are present in the blood serum. Antibodies interact with a particular antigen and causes agglutination. Agglutination is the clumping of foreign substance, that is antigen after interaction with antibody.