When a precipitation reaction is converted to an agglutination reaction by increasing the size of the antigen, what is the test referred to as?

In the context of immunology, when a precipitation reaction transitions into an agglutination reaction due to an increase in the size of the antigen, the relevant test is referred to as indirect agglutination. This process occurs when soluble antigens, which may normally lead to a precipitation reaction when bound to antibodies, are instead bound to larger carrier particles, such as latex beads or cells.

In this scenario, the antibodies can cross-link the antigens on these larger particles, leading to visible clumps or aggregates, which is characteristic of an agglutination reaction. This is particularly useful in laboratory diagnostics, where visual confirmation of a reaction can facilitate the identification of pathogens, antibodies, or other significant targets in a sample.

The other options do not accurately reflect this transition. Direct agglutination involves the agglutination of antigens that are themselves present in larger forms, rather than modified through the addition of carriers. A precipitation test typically involves soluble antigens and their corresponding antibodies but does not involve particles large enough to cause agglutination. The complement fixation test assesses the activity of complement proteins in the presence of antigen-antibody complexes, which is a distinct mechanism unrelated to the conversion of precipitation to agglutination.