The magic “red line” — a brief introduction to the principle of novel coronavirus antigen detection

Morphological structure of novel coronavirus

The virus is like a mixed jelly wrapped in a gum.

       Rubber candy sandwich “is located in the center of the virus body, and the main component is nucleic acid (ssRNA), which provides genetic information for the replication, inheritance and variation of the virus. The outer layer of “soft candy” is the protein shell of the virus body, called “nucleocapsid” (n), which can protect the virus nucleic acid from external factors and mediate the virus to enter the host cell. Before the entry, the “sugar coating” wrapped around the “soft candy” can be understood as the envelope of the virus body, which is a double-layer membrane wrapped around the nucleocapsid of the virus.

      This layer of “sugar coating” of novel coronavirus is extremely gorgeous. Its membrane contains spike glycoprotein (s), envelope protein (E) and membrane protein (m)

What does the “novel coronavirus antigen test” detect?

      Among these structural proteins, N protein is highly conserved among different sars-cov-2 virus strains and has good immunogenicity. It is the structural protein with the highest expression after the virus infects cells and is a good biomarker for detection. At present, the antigen detection of novel coronavirus is mainly based on the detection of N protein

Principle of antigen detection of novel coronavirus

     A white board, drop several drops of liquid into the sample hole, why can there be a magic “red line” after a few minutes?

     This antigen detection technique is called “colloidal gold method”. Take the novel coronavirus (2019 ncov) antigen detection kit that has been listed in China as an example, let’s take a “shallow” look at the formation process of the “red line”.

     It is assumed that this is a oropharyngeal / nasopharyngeal swab with novel coronavirus specific antigen (N protein). After the swab treatment solution is released and eluted, the N protein is dropped into the sample well with the treatment solution.

      The sample to be tested flows from one end of the test paper to the other end under the pull of the capillary force formed by the paper fiber. In this process, N protein will go through three hurdles.

        Remarks: the first level: gold labeled antibody (gold labeled 2019 ncov antibody); The second level: T-line antibody (2019 ncov antibody); The third level: c-line antibody (“gold label 2019 ncov antibody” secondary antibody)