3 Laboratory investigation
This week you will take your laboratory investigation one step further. The aim is to identify anyone who has been recently infected with COVID-19. Recall that last week you identified 7 serum samples that had antibodies against SARS-CoV2 spike protein and nucleocapsid, implying that they had been infected with the virus.
Of the 60 samples available those that have both S-antibodies and N-antibodies are:
C3111, C5930, F7812, H1151, H4439, M6723, N9921
Also recall that the first antibodies to be produced following infection are IgM, and it is only later that antibody production switches to IgG and IgA. Therefore, if you can identify sample(s) which have a relatively high IgM titre compared with IgG and IgA, it indicates that the person has been recently infected.
Activity _unit7.4.1 Activity 2 Measurement of S-antibodies of different classes
In this activity, we ask you to measure the titres of IgM, IgG and IgA S-antibodies in the 7 samples from infected individuals. You should also include the standard in the 8th row of the plate. The assay will require three ELISA plates, one for IgM, one for IgG and one for IgA.
ELISA; epidemiology on-line laboratory
Carry out the assay according to the protocol you developed in week 3, with the following conditions:
At step 1 of the assay you should select the samples; C3111, C5930, F7812, H1151, H4439, M6723, N9921 and the standard
At step 3 of the assay you should choose plates sensitised with spike protein.
At step 4, you will need to use 20µl anti-human IgM (HPO), 0.5mg/ml (final conc. 1µg/ml)
Or 4µl anti-human IgG (HPO), 1.5mg/ml (final conc. 0.6µg/ml)
Or 40µl anti-human IgA (HPO), 1.0mg/ml (final conc. 4µg/ml)
[The S-antibody titres of the standard are IgM-120, IgG-1200 and IgA-140.]
Record you results in a table like the one below (Table 3).
| Samples | S-antibodies | ||
|---|---|---|---|
| IgM | IgG | IgA | |
| C3111 | |||
| C5930 | |||
| F7812 | |||
| H1151 | |||
| H4439 | |||
| M6723 | |||
| N9921 | |||
| Standard | |||
You should see that one of these samples has a relatively high IgM titre in comparison to IgG and IgA. This suggests that the person has had a relatively recent infection with SARS-CoV2. When you have examined your data and come to a conclusion, click to check your answer.
Answer
Sample C5930 has IgM-160, IgG-120, IgA-80. The relatively high IgM titre suggests a recent infection. For the other samples the IgG titres are at least 8x higher than the IgM titre.