How to prevent immune cells from attacking transplanted head?

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One can't just filter something from blood while it goes to brain. Dialysis spends 3-6h to do that. One can't (now) do that in a second.

Suppose the translanted head is connected to genetically modified animal body via long vessels.

(I) Of course there are multiple immunosuppression strategies.
Probably we can even reduce bad influence of immunosuppressors on BBB by filtering those immunosuppressors from blood on its path to brain and returning immunosuppressors back when blood flows from brain to body.

1) Can we add some inhibitor of immune cells in the path of blood to brain, and remove it in the path of blood from brain?
2) Can we inactivate immune cells using optogenetics during their path to brain, and reactivate during their path from brain?
Not sure that such (a) fast (b) repeated 10k-100k times activation/inactivation is possible
3) Can we just somehow filter immune cells to prevent them from getting to vessels going to transplanted head? Human T-cells and B-cells size is 7-8μm[1] while red blood cells are 7.5-8.7μm in diameter[2]. So can't filter based on size.
4) Can we try to create two separate circles of blood circulation? One (without pig immune cells) for supplying transplanted head, another for