Hi Linyuan,
I'm afraid there is no easy way to explain all this without using a lot of jargon that make it difficult to understand, but I will do my best.
Although most people have heard of the ABO and the Rh Blood Group Systems, there are, in fact, 30 different Blood Group Systems. Within most of these systems, there are several antigens (mostly sugars or proteins that are expressed on the surface of the red cell). For example, within the ABO Blood Group System, there are three antigens (A, B and A1 - group O individuals have no ABO antigens), however, within the Rh Blood Group System there are well over 40 different antigens.
The Mi(a) antigen is found within the MNS Blood Group System (it was the 7th antigen found within this system, which also has over 40 different antigens).
Altogether, there have been something over 350 different human red cell antigens described, although, of course, no everyone expresses everyone of these antigens on their red cells.
If you lack a particular antigen, you can make antibodies against this particular antigen (alhtough you do not make antibodies against every antigen that you lack. Very often, although not always, you have to be stimulated to produce these antigens by either having been given blood that expresses an antigen that you lack, or by carrying a baby whose red cells express an antigen that you lack (expressed because the baby inherits a gene from the father that leads to the expression of this antigen). Sometimes, however, the body produces antibodies without any apparent stimulation (such as anti-A and/or anti-, although, in reality, there are stimulants within the environment.
Now, to try to answer your questions.
1) No, there is ABSOLUTELY nothing wrong with your blood, in terms of you producing the anti-Mia. Many millions of people throughout the world produce red cell antibodies, and they are as fit as a fiddle.
2) All antibodies are proteins. There are five basic types of these (IgA, IgD, IgE, IgG and IgM). All of these basic types produce lots of different specific antibodies - in your case, the specificity is anti-Mia.
The two types we are interested in, in the world of transfusion, are IgG (which are a sort of Y shape) and IgM (which are a sort of star shape). What we are looking at, in the laboratory, is the ability for these antibodies to cause clumping (the correct word is actually agglutination) of red cells that express the corresponding antigen. In your case, your anti-Mia will cause Mi(a+) red cells to clump, but not Mi(a-) red cells to clump.
IgM antibodies tend to cause this clumping of red cells without any help.
IgG antibodies, however, are smaller than IgM antibodies, and cannot "reach across" between two red cells to cause this clumping without a "bit of help". All antibodies, as I said are proteins, and all of these are a particular kind of protein called immunoglobulins. In the case of your anti-Mia, it would appear to be an IgG antibody (from the report), and one way that we can "help" the IgG antibodies to "reach across" two red cells to cause the clumping is to perform a particular test called an indirect antiglobulin test (I won't go into details - it is fairly comlpicated to explain, but is a very sensitive test), but for this test we use a particular reagent called anti-human globulin, and this is the AHG to which the report refers. So, if you require a blood transfusion, the laboratory would test (cross-match) the proposed units by this test, using AHG, to ensure that the blood is compatible with your antibody (i.e. your antibody would NOT cause the red blood cells to clump).
If your antibody did cause the red cells to clump, and you were transfused these red cells (don't worry - this blood would NOT be given to you - that is the point of the test), then your antibody could destroy these transfused red cells, and you could become very ill indeed (I STRESS AGAIN, the point of testing the blood to be transfused to you is so that this does not happen).
Haemolytic disease of the newborn happens when you have an IgG antibody of some specificity (in your case anti-Mia) and the foetus's red cells express the corresponding antigen (in your case Mia). The antibody can cross the placenta (IgM antibodies do not) and can destroy the foetuses red cells (haemolyse them). This used to be a real problem, but nowadays, as long as the pregnancy is well-monitored and, if considered necessary for the baby's health and wellbeing, there are such interventions as early delivery or intrauterine transfusions, then the baby will be born with no problems. I must stress, however, that many women have produced red cell antibodies, of many different specificities, and have had babies that are totally unaffected, despite the fact that they express the corresponding red cell antigen on their red cells - so don't worry!
I hope this explanation helps to some degree, but, if not, say so and I, or one of the many really nice people on this site, will try to explain it in a different way.