They also carry receptors for antibody molecules which allows them to reach out and grab foreign stuff, prior to destroying it. Passive immunity can also be acquired through the injection of Abs. In order for an immune response to be activated, an object must first be recognised as foreign. An immunogen is any molecule that stimulates an immune response. In general, proteins are the best immunogens, followed by carbohydrates and then nucleic acids.
Lipids are very poor.
An antigen is any molecule that reacts with an immune response. Often, antigen and immunogen are the same thing. This includes the chewing up and presentation of antigens by the macrophages. When they do this, specific B and T cells are stimulated, they begin to diviide and produce either lymphokines or antibodies. This takes about 4-7 days.