Antibodies bind with antigen

It forms lectin bindings with antigen proteins or antigen-antibody complexes, in most cases and IgA is the antibody. 

Figure 17-3 The competitive ELISA. If soluble antigen is included during the primary antibody incubation, the soluble antigen will compete for antibody binding with the plate-bound antigen.

Species reactivity - The species where the antibody binds the antigen. If the antibody does not react to the species of your tissue or has not been tested against the species you are using, do not use it. Some vendors will send a sample to test for a new species. Alternatively, the species that the antibody does not react with might also be listed. This is important because not all antibodies will bind equally to antigens from different species. 

Fig. 7.10 ABC and TSA. To obtain the largest amount of signal for a single 1° antibody combine ABC and TSA methods. After the 1° antibody binds the antigen and the 2° antibody labeled with biotin binds to the 1° antibody. The third incubation is with the ABC reagent that labels with an HRP. In a fourth incubation, HRP activation of biotin-labeled tyramide near the antigen, adds numerous biotins to the tissue near the antigen. The labeling is completed with fluorescence (Fig. 7.11) or enzymes (Fig. 7.12)...

Fig. 15.2 Silver enhancement of small gold, (a) The 1° antibody binds the antigen and the 2° antibody is labeled with small gold, (b) The silver enhancement solution was applied containing ionic sUver and a chemical developer, (c) The reaction deposited metallic silver on the gold particle.

Antibodies bind to antigen in liquid phase. They compete with binding of antibodies for solid phase antigen. 

Antibodies do not bind to antigen. They do not compete with antibodies binding with solid phase antigen. 

---