Your immunologist will only recommend starting immunoglobulin if you have had tests which confirm it is the right treatment for you. In cases of severe immune deficiency, only a couple of blood tests are required before the doctor will recommend immunoglobulin.
Fortunately, most people have mild immune deficiency and in this situation the doctor might try other treatments before immunoglobulin. For example, the immunologist might try giving you vaccines and checking how well you respond. This ‘vaccine challenge’ can take several weeks, as you have to have the vaccine, wait a few weeks before the blood tests and then get the results.
Your immunologist might suggest taking regular antibiotics for a few months and seeing how well these protect you from infection. This can be done whilst you are waiting the results of a vaccine challenge.
Finally, your immunologist might suggest trying immunoglobulin for a period of time, for example a year. If it is clear that you have benefitted, then they will recommend you continue it, but if you do not benefit your immunologist will suggest stopping.