Antibodies are produced by plasma cells (mature B-cells) with specific binding affinity for surface proteins that have been presented by antigen-presenting cells, like dendritic cells and macrophages. Once the plasma cell is stimulated by the presence of the specific antigen, it increases production of its antibody. These antibodies enter the blood and bind the antigen molecules on the surface of the pathogen cell. Cytotoxic T-cells and cytokines can then interact with the antibodies to initiate lysis of the infected cell or pathogen.

It helps to think of antigens and antibodies like a lock and key: they are highly specific for one another. B-lymphocytes create only one type of antibody. When this antibody attaches to an antigen presented by a macrophage or antigen-presenting cell, the B-lymphocyte will differentiate with the help of a helper T-cell. The result is replication of the B-lymphocyte to produce more of the same antibody from plasma cells and generate memory B-cells to easily respond in the event of a second infection by the pathogen.

The question specifies that the antibodies in question are serum antibodies, while IgA is primarily an antibody type secreted into luminal environments. As a result, we can conclude that IgA is not likely to be involved at all in this experiment. Beyond this, you must know that IgM is the type of antibody that is produced upon initial pathogen exposure. After CD4 cells facilitate isotype switching, IgG is produced and demonstrates more robust binding.

The epitope is the region of a target cell to which an antibody binds. The remaining choices are all structural regions of the antibody itself, as opposed to the target cells to which an antibody binds.

IgG is the only class of immunoglobulin that is able to cross the placenta. This is important as this immunoglobulin is able to provide passive immunity to the unborn fetus

MHC I is found on all nucleated cells and presents antigens to cytotoxic T lymphocytes. These cytotoxic T cells contain CD8 receptors, which binds to MHC I. MHC II cells are found on B-lymphocytes and antigen presenting cells only. MHC II presents antigens to helper T cells, which contain CD4 receptors. 

IgE is able to bind via the (fragment crystallizable) Fc region to mast cells and basophils. This binding allows these cells to release their granule contents involved in many allergic reactions. 

IgM is the first antibody produced during the primary immune response. It is the only immunoglobulin whose shape is a pentamer. IgG is the most abundant immunoglobulin class in the blood and tissue and has the ability to cross the placenta. IgE binds to basophils and mast cells and is involved in the allergic response. 

For any recipient, we must consider what antibodies they have. A patient with A positive blood has the following antibodies: anti-B For any donor, we must consider what antigens they have. The donor cannot have an antigen that matches the recipient's antibodies, or else agglutination will occur. Therefore, any B blood types will result in agglutination. The O positive person only expresses the Rh antigen, and the recipient does not express this antibody. No agglutination will occur.

A person with O negative blood is known as the universal donor because their blood cells completely lack antigens. This means their blood cannot be recognized as foreign by a donor. For blood transfusions, we must keep in mind the recipient's antibodies. For an O negative recipient, he/she produces A and B antigens, and Rh antigens if they have been previously exposed to Rh positive blood. That means they cannot accept blood from donors with A, B or possibly Rh positive blood. They must receive O negative blood only.