Immunogenicity

[dttom]

I am studying mammalian immunology this summer holiday, and I am reading a relevant book. It is said that immunogenicity of a substance depends partly on its molecular size and chemical complexity; assume other things being equal, a foreign substance of molecular size higher than 100000 Da or above tends to be better immunogen that which below; and a chemically more complex substance tends to be a better immunogen than which simpler. However, there is no further explanation about these, I would know why such 'rules' determine, though partly, the immunogenicity of a substance. Thanks.