Antibodies raised against a macromolecular antigen usually produce an antigen-antibody precipitate when mixed with that antigen. Explain

Question:

Antibodies raised against a macromolecular antigen usually produce an antigen-antibody precipitate when mixed with that antigen. Explain why no precipitate forms when
(a) Fab fragments from those antibodies are mixed with the antigen;
(b) Antibodies raised against a small antigen are mixed with that small antigen; and
(c) The antibody is in great excess over the antigen and vice versa.