What type of procedure is it when the antigen is a natural component of the infectious entity?

The direct agglutination procedure is characterized by the use of antigens that are natural components of the pathogens themselves. This type of assay involves mixing a suspension of the infectious organism (which contains the antigen) with a sample containing specific antibodies. When these two are combined, if the antibodies specific to the natural antigen are present, they will bind to the antigens, causing visible clumping or agglutination.

In contrast, indirect agglutination relies on the use of artificially coated particles or cells that are conjugated with an antigen. Competitive inhibition assays involve the competition between a known antigen and the sample antigen for binding to antibodies, which is more complex than a straightforward direct interaction. Western blot analysis is a technique used to identify specific proteins in a sample rather than for the direct detection of pathogens using agglutination techniques.

Therefore, the direct agglutination procedure is the correct choice as it directly involves natural antigens present in infectious organisms and is designed to assess the presence of corresponding antibodies in a simple and effective manner.