An inflammatory response that occurs immediately upon exposure to antigen is MOST LIKELY to be mediated by:

The inflammatory response that occurs immediately upon exposure to an antigen is most likely to be mediated by IgE antibodies.

Here's a step-by-step explanation:

1. When the body is exposed to an antigen (a foreign substance), the immune system is triggered.

2. The immune system produces antibodies to fight off the antigen. These antibodies are produced by B cells, a type of white blood cell.

3. There are several types of antibodies, but the one that is most likely to mediate an immediate inflammatory response is IgE.

4. IgE antibodies are involved in allergic reactions. They bind to the antigen and trigger the release of histamine and other chemicals from mast cells.

5. These chemicals cause the symptoms of inflammation, such as redness, swelling, and heat. This response happens quickly, often within minutes of exposure to the antigen.

So, the answer to your question is IgE antibodies.