Sensitivity can be considered at different levels in the total of preparation and incubations. Ideally during preparation one would like to preserve all antigens present. In many cases this is not possible. But at least a representative fraction should be preserved and be available for immuno labeling. It all depends on the preparation procedure (fixation, embedding, temperature, etc.), which leaves you with a specimen or section with a given number of available and recognizable antigens. The ensuing detection protocol has 100% sensitivity if all the remaining antigens are detected, i.e. are represented by at least one gold particle or marker molecule. Again, due to masking and steric hindrance by the specimen composition this will only in exceptional cases be fully attained. The immuno labeling sensitivity thus expresses the degree to which available antigens can be detected by the employed combination of primary antibody and secondary conjugate.
The quality of the primary antibody is the next important item. Theoretically the Kd-value of an antibody/antigen reaction is a measure for the dilution at which the incubations should be performed and for the stability of the ensuing bond. Sensitivity will go up with more concentrated antibody solutions up to a maximum level. However, when the primary antibody shows cross-reactivity there is not necessarily an improved signal-to-noise ratio. The reliability of the detection by the primary antibody improves in such cases with higher dilutions, probably leading to a smaller amount of antigens detected, but to an improved signal-to-noise ratio. Thus, sensitivity at the level of the primary antibody has to be balanced against the signal-to-noise ratio.
The last step is the quality of the secondary reagent. In fact you will be looking at a number of gold particles which represents a number of secondary antibodies which have detected a number of primary antibodies. For the interaction between the secondary reagent and the primary antibody the same rules apply as indicated for the antigen/primary antibody reaction.
Detectability reflects the degree to which the final result of all the reactions involved can actually be seen. This is depending on the right match between particle size and magnification. Ultra small particle-based conjugates for instance are among the most efficient detection systems, but you will only detect them after silver enhancement (in most applications).