I remember (not clearly) that AABB states for neonate less then 4 months, we should give type O washed red cells.

If I remember rightly, but the IgG antibody' s half life is more than 20 days, then why we need 4 months to avoid the maternal sourced antibodies?

I post this question today, it is because I just met an ABpos baby,he is 3 months old, and still has anti-A  in his blood, his mom is Bpos.

Someone had transfused him with AB pos packed red cells yesterday, and I can still detect the anti-A in his plasma. I think it was stronger before transfution.

So, the questions confused me are

1. B mom produce IgG anti-A , I used to think only O type produce IgG anti-A/B

2. 3 months the maternal antibodies are still here, so the half life of IgG is longer than 20 more days

1.  By no means are ABO IgG antibodies only produced by group O people.  Certainly, they are most commonly found in group O individuals, but they are not exclusive to group O individuals.

2. The half life just means that, after 20 days (I've always been taught 21 days, but 1 day makes no difference), instead of there being 16g/L, there will be 8g/L.  After the next 20 days, there will be 4g/L and so on and so forth.  However, because IgG molecules are actively transported from the maternal circulation into the foetal circulation, particularly in the third trimester, at birth, the concentration of IgG in the baby's circulation is often higher than that in the maternal circulation, and so the half-life would appear to be longer (in reality, it isn't).  Lastly, only 52% of the IgG will be in the circulation - the other 48% is in the interstitial fluid - although this is the same for babies and adults.

I don't think this is an AABB rule else we would be adhering to it. What we give is O= or O+ red cells less than 5 days old if large volume(>60 ml) or less than 28 days for smaller amounts. Once in a while we have received an request for an A, B or AB because that is what the baby types and that is what Daddy or other donor is and the family wants Direct Donor. We then do a crossmatch on the baby through IgG to detect incompatibility due to Mom's antibody (A,B AB). 

AABB does not require or recommend that you give group O washed blood to neonates < 4 months of age.  I work at an AABB accredited children's hospital (so lots of neonate transfusions) and we have never used washed blood for them during the 28 years I've worked here. 

31st Edition of AABB Standards for Blood Banks and Transfusion Services:

5.17.2  If a non-group-O neonate is to receive non-group-O Red Blood Cells that are not compatible with the maternal ABO group, the neonate's serum or plasma shall be tested for Anti-A or Anti-B.

          5.17.2.1  Test methods shall include an antiglobulin phase using either donor or reagent A1 or B red cells. Std. 5.14.3.4 applies.

          5.17.2.2   If anti-A or anti-B is detected, Red Blood Cells lacking the corresponding ABO antigen shall be transfused.

Our policy is that we routinely give neonates fresh group O Leuko-reduced, Irradiated, CPDA RBCs, but if they must be given something other than group O (example: a directed donor unit), then we require a full AHG crossmatch be done on the unit using baby plasma. 

I am sorry about the mistake I have made. I remember it wrong.