What is the strength of a visible reaction known as?

The strength of a visible reaction in immunology is referred to as avidity. Avidity represents the overall strength of binding between an antibody and its antigen, taking into account both the affinity of individual binding sites and the valence, or the number of sites that can bind the antigen. This concept is crucial in understanding how antibodies interact with antigens in a functional way, particularly in situations where multi-valent binding enhances the stability and strength of the antigen-antibody complex.

Specificity relates to the ability of an antibody to selectively bind to a particular antigen among a mixture of substances, but it does not quantify the strength of that binding. Affinity describes the strength of the interaction between a single binding site of an antibody and its specific antigen, but it does not consider multi-site interactions that can increase the overall binding strength. Complementation does not apply directly to the context of binding strength but rather refers to the process by which proteins or other molecules can work together to exhibit a functional effect or represent a biological concept in other areas of immunology.

Thus, avidity is the term that best encompasses the overall strength of the visible reaction observed in antigen-antibody interactions.