Heating it or freezing it will not kill the antibodies (they are proteins). It may cause them to be denatured some but they are still reactive and effective. You do not want to heat it up to the point of boiling but warming it is fine. We freeze antibodies all the time in the lab and they are fine.
You'll create hot spots that could burn your little one's mouth. Just heat water in the microwave (or fill with hot water from the faucet) and put the bottle in the water. Much safer!
You should never heat breast milk or formula in the microwave because it can create hot spots which can burn baby's mouth. It also does hurt the nutrition quality.
From the CDC:
Safely Thawing Breast Milk
As time permits, thaw frozen breast milk by
transferring it to the refrigerator for thawing or by swirling
it in a bowl of warm water.
Avoid using a microwave oven to thaw or heat bottles of breast milk
Microwave ovens do not heat liquids evenly. Uneven heating could easily scald a baby or damage the milk
Bottles may explode if left in the microwave too long.
Excess heat can destroy the nutrient quality of the expressed milk.
Re: BM antibodies question
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From the CDC:
Safely Thawing Breast Milk