or Is it required to use secondary antibodies raised in the same species as blocking serum for IHC? Thanks.
You should be fine. The block as same as secondary is to minimize chances of secondary antibody reacting with the block. Chance exists you will have some cross-reactivity. So if you have a really low expressing protein or a secondary that isn't very good, you may have problems. But, as I said, you should be fine.
Your IHC should work just fine. Like Matthew said, using the same block as secondary is mostly to minimise any cross-reactivity from the secondary antibody. As long as there's a large mix of proteins to block the non-specific binding sites, you should be fine. I typically use 5%NGS with a pinch of BSA just to add that extra randomisation of proteins.
Best of Luck
As long as your secondary is not Donkey anti Goat you will be fine. If you used a DaG secondary you would have a lot of nonspecific staining from interactions with the serum.
The blocking step is to saturate non-specific protein binding sites in your samples as well as in your antibodies (mostly secondary, but primary Abs also contain non-specific binding sites). Use of normal serum from the same species as the secondary Ab (donkey) is the most appropriate (though horse serum would also do). Of course, normal serum from the same species as the primary antibody (you didn't tell us if it is a goat or not) should not be used.
Otherwise, ready-to-use serum-free protein blockers are perfect substitutes for normal sera of any species. When regularly using antibodies from multiple species (including goat) a serum-free blocker would release you from remembering what type of normal serum should be chose for today experiment. The other convenience is that such blockers can be stored in fridge instead of freezer. Sometimes people also use skimmed milk as a cheep solution in blocking.
All the above stated suggestions are quite appropriate for blocking non-specific binding of the secondary antibody. However, there is always a possibility of getting non-specific immunostaining. Moreover, using avidin-biotin detection system could also be source of non-specific staining. To avoids this problem, you could use polymer-based detection system which does not require application of serum to circumvent non-specific staining. Such a detection system is widely available to purchase from various bio-tech companies. Please let me know if you would like me to recommend a few whose products I have used with excellent results.
