The primary disadvantage of the direct IHC method is its **lack of signal amplification and lower sensitivity**. In the direct method, the label (e.g., an enzyme or fluorochrome) is conjugated directly to the primary antibody. This single-step procedure is simple and fast, with low risk of cross-reactivity. However, only one label molecule is delivered to each antigen site. If the target antigen is present in low amounts ('low copy number'), the resulting signal may be too weak to detect reliably under a microscope. In contrast, indirect methods (like the two-step indirect or multi-step ABC/PAP methods) provide amplification. A single primary antibody can be bound by multiple secondary antibodies, each carrying a label, or by large complexes containing many enzyme molecules. This multiplies the signal at each antigen site, making it possible to visualize antigens that are scarce. Therefore, for most diagnostic applications where sensitivity is paramount, indirect methods are preferred over the direct conjugate method.

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