explain rhesus factor

[Oh_Gee]

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How does rhesus factor work in pregnancy? i am confused as to how a mother can be negative for it but her fetus can be positive from it? inherited from father?

 

[techfan]

The RHD gene is dominant so a person is considered to be RhD positive whenever this gene is present, even though the gene may have been inherited from one parent.

The mother will build up immune cells to attack the Rh factor in the fetus (since she views it as foreign). In the first child, it takes time to build up these factors, so the first kid will be born before the mom's immune cells can attack. For the second kid, the immune system will be ready and will attack as soon as it sees that the fetus' blood has the Rh factor exposed (blood has to mix so that the fetus can get oxygenated blood).

 

[The Brown Knight]

Rh factor works by classical dominance (whereas ABO group works by codominance), so yeah inherited from father. It's the +/- associated with your blood type: B positive is someone with B type and presence of Rh antigen
If fetus has Rh antigen but mother doesn't and because the mother's immune suppressor cells near the womb don't entirely restrict antibodies from being transported between fetal and maternal blood, the mother's Rh antibodies can transfer into the fetus and eventually promote hemolysis in the fetus.

 

[Czarcasm]

The RHD gene is dominant so a person is considered to be RhD positive whenever this gene is present, even though the gene may have been inherited from one parent.

The mother will build up immune cells to attack the Rh factor in the fetus (since she views it as foreign). In the first child, it takes time to build up these factors, so the first kid will be born before the mom's immune cells can attack. For the second kid, the immune system will be ready and will attack as soon as it sees that the fetus' blood has the Rh factor exposed (blood has to mix so that the fetus can get oxygenated blood).

Sorry, just a small point of clarification - for the mom and fetus, blood rarely ever mixes. Instead, nutrients including oxygen diffuses across the placenta. The mother's own IgG antibodies can diffuse across the placenta (a special characteristic of these antibodies) and so, upon exposure to the antigen, the mother will develop IgG's against the Rh antigen that can attack the second baby. Clinically, mothers who are Rh- can be given a Rhogam injection (immediately after birth, because some of the fetus and mother's blood come into contact) and basically this prevents the activation of naive B cells that recognize the Rh antigen from maturing to antibody producing plasma cells. Unfortunately, mothers who neglect being tested put both themselves and also, the baby at risk to develop Rh disease (erythroblastosis fetalis).

 

[Oh_Gee]

Sorry, just a small point of clarification - for the mom and fetus, blood rarely ever mixes. Instead, nutrients including oxygen diffuses across the placenta. The mother's own IgG antibodies can diffuse across the placenta (a special characteristic of these antibodies) and so, upon exposure to the antigen, the mother will develop IgG's against the Rh antigen that can attack the second baby. Clinically, mothers who are Rh- can be given a Rhogam injection (immediately after birth, because some of the fetus and mother's blood come into contact) and basically this prevents the activation of naive B cells that recognize the Rh antigen from maturing to antibody producing plasma cells. Unfortunately, mothers who neglect being tested put both themselves and also, the baby at risk to develop Rh disease (erythroblastosis fetalis).

so blood only mixes during birthing? and Rhogam is specific for Rh naive B cell or all naive B cells?

 

[Czarcasm]

so blood only mixes during birthing? and Rhogam is specific for Rh naive B cell or all naive B cells?

Generally, that's the case - things diffuse through the placenta but the mother and developing baby have their own capillary bed within the placenta that allows diffusion to occur. As for the specifics of Rhogam, I cannot remember exactly the mechanistic detail, but what I do know is that it acts specifically on the naive B cell that has a BCR specific for that antigen. The BCR of that naive antigen will bind to the Rh antigen but then Rhogam acts to suppress the activation from ever happening. In its absence, the naive B cell would mature to plasma cell. Also, naive B cells in general act specifically and uniquely to a given antigen, so only naive B cells that recognize the antigen will be targeted for suppression. The others are not involved and so, don't really pose a problem anyways.

 

[techfan]

Thanks, I've been curious on how that worked.

 

[Oh_Gee]

Generally, that's the case - things diffuse through the placenta but the mother and developing baby have their own capillary bed within the placenta that allows diffusion to occur. As for the specifics of Rhogam, I cannot remember exactly the mechanistic detail, but what I do know is that it acts specifically on the naive B cell that has a BCR specific for that antigen. The BCR of that naive antigen will bind to the Rh antigen but then Rhogam acts to suppress the activation from ever happening. In its absence, the naive B cell would mature to plasma cell. Also, naive B cells in general act specifically and uniquely to a given antigen, so only naive B cells that recognize the antigen will be targeted for suppression. The others are not involved and so, don't really pose a problem anyways.

so if a RH negative mother already produced RH antibodies and had another RH positive fetus in her, would treatment in this case be to give antibodies binding the RH antibodies in her (or something along those lines?)

 

[kraskadva]

A Rh neg mother who has been pregnant before, regardless of outcome (term, miscarriage or abortion) since all can involve mixing of maternal & fetal blood, will be given Rhogam from early in the pregnancy (~end of 1st trimester if I remember correctly) to prevent issues.
IDK exactly the mechanism for Rhogam, but a google search would probably help you here.

 

[Czarcasm]

so if a RH negative mother already produced RH antibodies and had another RH positive fetus in her, would treatment in this case be to give antibodies binding the RH antibodies in her (or something along those lines?)

The treatment isn't to give antibodies (we're trying to prevent them from ever forming). In the absence of Rhogam treatment after exposure (Rh- mother to Rh antigen), the mother will develop antibodies and at that point, any baby she gives birth to in the future is at risk. There is no treatment (as far as I'm aware) to prevent this from happening - at that point it's too late.

 

[kraskadva]

http://en.wikipedia.org/wiki/Rho(D)_immune_globulin

The medicine is a solution of IgG anti-D (anti-RhD) antibodies that take out any fetal RhD-positive erythrocytes which have entered the maternal blood stream from fetal circulation, before maternal immune system can react to them, thus preventing maternal sensitization.[1] In a Rhesus-negative mother, Rho(D) immune globulin can prevent temporary sensitization of the maternal immune system to Rh D antigens, which can cause rhesus disease in the current or in subsequent pregnancies. With the widespread use of Rho(D) immune globulin, Rh disease of the fetus and newbornhas almost disappeared. The risk that a D-negative mother can be alloimmunized by a D-positive fetus can be reduced from approximately 16% to less than 0.1% by the appropriate administration of RhIG.