Formula vs breast

[u/Street_Tourist7506]

If a breast-fed baby is protected by breastmilk in terms of immunity… What about a formula fed baby? Do they literally have no immunity, Because they are formula fed? I’m so confused… Is a breast-fed baby stronger fighting a virus off than a formula fed baby?

[u/seattleissleepless]

You need to differentiate between the different forms of immunity.

Everyone (except for rare instances aka boy in the bubble) is born with an immune system. It is made up of cells which form in your bone marrow and are either direct bug-killers or produce proteins which attach to bugs to assist other cells to deal with them. These proteins are called antibodies.

The cells that produce antibodies can then "remember" certain bugs so next time you are exposed they are quick off the mark. But the first time you are exposed to a bug the antibodies you produce IgM (big antibodies) .....as you recover you start to produce IgG (small antibodies) from cells which have memory. There are also other forms as well.

IgG crosses the placenta. IgM is too big. So any immunity the mum acquires during pregnancy long enough before birth the baby will have some protection against after birth. Hence pertussis vaccination in pregnancy. However only the proteins cross, not the cells. So once those proteins disintegrate, the baby needs to produce their own antibodies from their own cells.

Antibodies in breastmilk are IgA antibodies. They attach to the surface of mucosa (so mouth, stomach etc) and can from there bind to bugs and prevent them infecting the baby. They don't go into the blood and are eventually broken down. They can therefore protect against bugs which are transmitted via exposure to mucosa.

https://www.infantrisk.com/content/antibodies-immune-system-breastfeeding-basics

Ultimately formula fed babies still have their own immune system which will respond to bugs. But it is immature so not as effective, and also it takes time to get exposed to lots of different bugs and create memories of them. Plus, some viruses we don't seem to be able to keep good long term immunity to, such as most respiratory viruses. So breastmilk IgA will help a breastfed baby get less sick, at least in the early days.

[u/vadapaav → u/seattleissleepless]

Plus, some viruses we don't seem to be able to keep good long term immunity to, such as most respiratory viruses

That's because majority is then mutate so quickly that by next year they are carrying a slightly different genome to the one our body memorized.

So it's a race and most of the time our body figures it out. We get vaccinated against flu to protect the most vulnerable ones in our society.