Agglutination and precipitation both involve cross-linking; however, precipitation measures what type of antigens?

Precipitation involves the formation of visible complexes when soluble antigens react with soluble antibodies. In this process, the soluble antigen-antibody complexes aggregate to form larger complexes that can settle out of solution, making them easier to detect. This phenomenon occurs in a liquid environment where both antigens and antibodies are free to interact.

In contrast to whole cells or particulate matter, which are involved in agglutination reactions, precipitation specifically applies to soluble antigens. When the right concentrations of antibodies and antigens are present, they can form a precipitate, indicating a positive reaction. This characteristic distinguishes precipitation from agglutination, where the cross-linking leads to clumping of whole cells or particles rather than forming a settled precipitate from solution.

Understanding this distinction allows for better interpretation of serological tests and assays, as the nature of the antigens being tested—whether they are soluble or insoluble—determines the appropriate method to use for their detection.