In a precipitation reaction, the antigen examined is a(n) molecule.

In a precipitation reaction, the antigen being examined is indeed a molecule. This type of reaction is commonly used in immunology to study the interaction between an antigen (a foreign substance that induces an immune response) and an antibody (a protein produced by the body's immune system to neutralize the antigen).

Here are the steps involved in a precipitation reaction:

1. Mixing Antigen and Antibody: The first step in a precipitation reaction is the mixing of the antigen with its specific antibody. This is usually done in a liquid medium.

2. Formation of Antigen-Antibody Complex: Once the antigen and antibody are mixed, they bind to each other to form an antigen-antibody complex. This is a reversible reaction, meaning the complex can dissociate back into the individual antigen and antibody.

3. Formation of Lattice Structure: If the antigen-antibody complex is present in optimal proportions, they can cross-link with each other to form a lattice structure. This is the actual "precipitate" in the precipitation reaction.

4. Visible Precipitate: The lattice structure grows large enough to become visible as a precipitate. This can be seen as a cloudy or opaque appearance in the liquid medium.

5. Sedimentation: Over time, the precipitate settles at the bottom of the container due to gravity. This is known as sedimentation.

So, in summary, the antigen in a precipitation reaction is a molecule that is specifically recognized and bound by an antibody, leading to the formation of a visible precipitate.