Video Transcript
The flowchart shows the series of events that can occur if a rhesus-negative mother is pregnant with a rhesus-positive child. What would the most likely statement to complete the flowchart be? (A) The child will inherit Rh-negative antigens from its mother and become Rh negative. (B) The mother will obtain Rh-positive antigens from the child and become Rh positive. (C) The antigens produced by the baby can attack the antibodies of the mother. Or (D) the antibodies produced by the mother can attack the red blood cells of the baby.
First, let’s review what some of these terms mean. Being rhesus negative or Rh negative means that the red blood cells do not have a rhesus antigen on their surface, while being rhesus positive or Rh positive means that an individual does have rhesus antigens on their red blood cells surface. We might remember that an antigen is a substance that causes an immune reaction. This includes the proteins on the surface of cells that function in cell identification or in helping the immune system to determine whether a cell belongs in the body or whether it’s a foreign cell, which might potentially be pathogenic or disease causing.
The rhesus antigen is one of these types of antigens that occurs on cell surfaces. Specifically, the rhesus antigen can appear on red blood cells. And an individual’s red blood cells can either have the rhesus antigen, in which case we call them rhesus positive or Rh positive, or not have the rhesus antigen on the red blood cells, in which case we call them rhesus negative or Rh negative. We should remember generally that an individual’s immune system will build antibodies against any antigens that are not self, that is, any antigens that are not expressed on an individual’s own tissues.
So, while an Rh-positive individual who naturally produces the Rh factor will not build antibodies against the Rh factor, an Rh-negative individual will build antibodies against the Rh factor. However, this will only happen if that individual’s immune system comes in contact with the Rh factor at some point, since the immune system only builds resistance against antigens it has encountered before. Typically, a rhesus-negative person’s immune system would not encounter the rhesus antigen inside their own body.
However, if a rhesus-negative individual becomes pregnant with the child of a rhesus-positive individual, the child may inherit the Rh antigen from the father and be Rh positive. In this case, the mother’s immune cells may encounter the Rh antigens being produced on the baby’s red blood cells via the placenta or during the birth of the child.
When this happens, the Rh-negative mother’s immune system will build antibodies against the Rh-positive child’s red blood cells. Because the primary or initial immune response to an antigen is slow, this first immune reaction is unlikely to harm the child. However, the next time that this mother is pregnant with an Rh-positive child, perhaps as the result of having another child with the same Rh-positive father, the secondary immune reaction at that time will be much more swift and strong and can potentially harm the developing child.
So, let’s look back at the question and the flowchart we’re provided with. We’re asked about a situation where a rhesus-negative mother is pregnant with a rhesus-positive child. The events in the flowchart are as follows. A mother is pregnant with an Rh-positive baby. Then, during the pregnancy, her body starts producing antibodies that recognize the foreign blood cells. This is called sensitization. Then, the mother gives birth. Then, if the mother becomes pregnant with a second Rh-positive baby, her body produces antibodies immediately. The last step in the flowchart is blank. So, let’s think about this situation.
We know that the Rh-negative mother will build antibodies against the first Rh-positive child’s red blood cells. And in fact, we are told that this happens in the flowchart. Because the mother’s immune response to this new antigen is slow, there is no effect on the first child. However, then the mother becomes pregnant with the second Rh-positive baby, and her body produces antibodies immediately. What will those antibodies do? They will attack the Rh-positive baby’s red blood cells. So, let’s look through our answer choices.
(A) says the child will inherit Rh-negative antigens from its mother and become Rh negative. However, in the flowchart, we are already told that the child is Rh positive. Whether an individual will express this antigen is genetically based, so if the child has inherited the Rh-positive characteristic from the father, then the child already has that characteristic, and it won’t change based on the environment that it’s in. So, we can eliminate (A). Likewise, the mother’s RH status is genetically based, so she will not become Rh positive by obtaining Rh-positive antigens from the child either. So, we can eliminate (B).
(C) says the antigens produced by the baby can attack the antibodies of the mother. But this is backward. Antibodies attack antigens, not the other way around. So we can eliminate (C). The correct answer is (D): the antibodies produced by the mother can attack the red blood cells of the baby.
