How do we know if our immune system is working properly? Are there any signs that indicate that there is a problem?

Ive had irritation in my throat for about 13 days now...Does this mean that my immune system could be weakened?

John,

Ive suffered simular symptons and saw my gp who gave me antibiatics (sic) and told me when I was finished to have a blood thest (he gave me a script) to check my immune system.