Precipitation differs from agglutination in which way?

• A. Precipitation can only be measured by an automated instrument.
• B. Precipitation occurs with univalent antigen, whereas agglutination requires multivalent antigen.
• C. Precipitation does not readily occur because few antibodies can form aggregates with antigen.
• D. Precipitation involves a soluble antigen, whereas agglutination involves a particulate antigen.

Answer: (D)

The main idea is that the form of the antigen determines the type of reaction. In precipitation, the antigen is soluble in solution, and antibodies link these soluble molecules into a lattice that grows until insoluble aggregates precipitate out. In agglutination, the antigen is attached to particles (such as cells or beads), providing multiple binding sites on each particle; antibodies cross-link these particles directly to form visible clumps. That makes precipitation a soluble-antigen phenomenon and agglutination a particulate-antigen phenomenon.

Other statements don’t fit: precipitation isn’t limited to automated instruments; it can be observed by manual turbidity or other simple readouts. Precipitation actually requires multivalent antigens to form a lattice, not univalent ones. And precipitation is not simply due to scarcity of antibodies but due to the soluble nature of the antigen forming a lattice that becomes insoluble.