What type of reaction is expected when soluble antigen is added to homologous antibody?

When soluble antigen is added to homologous antibody, a precipitation reaction is expected. In this scenario, the soluble antigen and the specific antibody form insoluble complexes that precipitate out of solution. This occurs when the ratio of antigen to antibody is optimal, allowing them to bind together effectively and create larger aggregates that can no longer remain in solution.

In precipitation reactions, the presence of both the soluble antigen and antibody leads to the formation of visible aggregates. This is an important principle in various immunological assays and diagnostics, as it can be used to quantify the concentration of antibodies or antigens in a sample.

For additional context, while agglutination involves the clumping of particles, such as bacteria or red blood cells, due to the presence of specific antibodies, it typically requires either particulate antigens or cells, not soluble antigens. Neutralization refers to the ability of antibodies to bind to a pathogen or toxin and block its biological activity, which is a different mechanism altogether. Complement activation relates to the immune response activated by the formation of antibody-antigen complexes, but it does not specifically describe what happens when soluble antigens interact with homologous antibodies.