我来回答你

toxoplasmosis的血液检查应该有两个指标，IgG和IgM，如果从来没有感染过，两项指标都是阴性。如果感染了，IgM会成阳性，但只会维持六个月到一年的时间，但IgG would remain positive

That means, if You find (stable) IgG and no IgM: there had been the infection longer time ago, and there is protection now (and in the future).

医生说我们家猫既然是户外的我应该免疫了....NO, you are not immuned to the cats.
Be careful when handling the cat litters. Better get your cats checked by the vet, if they don't have TOXO, make them indoor from now on to get a peaceful mind. Most likely, you cat does have TOXO, so just dedicate the little cleaning work to your hu*****and.