Rh negative people are not born with Rh antibodies. After exposure to Rh positive blood, the Rh negative patient will begin producing Rh antibodies. Any subsequent exposure will cause agglutination. This has applications in childbirth, if there mother is Rh negative, and the baby is Rh positive. It is possible for the baby's blood to come in contact with the mother's (especially during childbirth). Neither the baby nor the mother are at risk, however, if the mother has another baby that is Rh positive, the mother will have antibodies from when she first encountered Rh positive blood from the previous baby. This may cause harm to the second (and any subsequent) Rh positive babies.

Blood antigens are the protein markers on the surface of red blood cells. On an A positive red blood cell, there is the A marker (antigen) and the Rh marker (antigen). Antibodies are found in the blood plasma, and these bind to foreign antibodies to cause agglutination. People produce antibodies for the antigens they do not have (Rh antibodies are only made after exposure to Rh positive blood). An A positive person will express the B antibody.

Blood antigens are the proteins markers on the surface of a red blood cell. On an AB positive red blood cell, there is the A marker (antigen), the B marker (antigen) and the Rh marker (antigen). Antibodies are found in the blood plasma, and these bind to foreign antibodies to cause agglutination. People produce antibodies for the antigens they do not have (Rh antibodies are only made after exposure to Rh positive blood). An AB positive person will not have any antibodies, otherwise they would bind to their own red blood cells and cause agglutination.

We must first assume the two people are A, B, O compatible (ie., both patients are type A). An Rh negative person is negative because they lack the Rh antigen. An Rh positive person does not produce any Rh antibodies or else they would attack their own blood. Therefore, there is no antigen to attack and no antibodies to attack with, so agglutination will not occur.

Skin is an innate external defense barrier and does not involve antibodies. Inflammation and antimicrobial proteins are innate internal defense mechanisms, and are not pathogen-specific. T-lymphocytes are adaptive cell-mediated defense mechanisms. The humoral system, though, is part of the adaptive immune system, which delivers antibodies through blood to fight antigens extracellularly.

Antibodies are part of humoral immunity. The humoral pathway protects against extracellular pathogens. Antibodies are produced and secreted by B lymphocytes (B cells). They recognize free antigens, initiate activation of other immune cells, and coat the antigen for destruction (which may or may not be cellular). 

An AB positive patient is known as a universal recipient because they can receive blood from any blood type. The recipient's antibodies are what will attack foreign antigens. Type AB positive patients produce no antibodies, because any antibody produced would attack their own antigens, causing agglutination. Because type AB patients patients do not produce antibodies, they cannot attack any antigens and they can receive any blood type.