**Core Concept**
Hemagglutinin antibodies, such as Anti-A and Anti-B, are a type of antibody that plays a crucial role in the ABO blood group system. These antibodies are responsible for recognizing and binding to specific antigens on the surface of red blood cells, leading to agglutination or clumping of the cells.

**Why the Correct Answer is Right**
The correct answer is IgM because hemagglutinin antibodies, including Anti-A and Anti-B, are primarily of the IgM class. IgM antibodies are the first line of defense against infections and are characterized by their pentameric structure, which allows them to bind to multiple antigens simultaneously, resulting in efficient agglutination. The IgM class is particularly effective in recognizing and reacting with ABO blood group antigens on red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG antibodies are a secondary line of defense and are not primarily responsible for hemagglutination reactions. While IgG antibodies can also bind to ABO blood group antigens, they are not the primary type of antibody involved in hemagglutination reactions.
**Option C:** IgA antibodies are primarily involved in mucosal immunity and are not the primary type of antibody involved in hemagglutination reactions.
**Option D:** IgE antibodies are involved in allergic reactions and are not the primary type of antibody involved in hemagglutination reactions.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the ABO blood group system is a critical factor in transfusion medicine, and understanding the role of IgM antibodies in hemagglutination reactions is crucial for preventing adverse reactions in patients receiving blood transfusions.

**✓ Correct Answer: B. IgM**