We already know that microglia attacks the protein BA, which hurts healthy brain cells. The origin of protein BA doesn’t add any relevant information, since we already have enough in the premises to understand the relationship between microglia and the protein BA.

This strengthens by connecting the premise about acid to the conclusion. We know that microglia are the brain’s immune cells. If the acid can slow deterioriation, and it also reduces microglia, that suggests microglia might have a role to play in causing deterioration.

Even if microglia decreases the buildup of protein deposits, we already know how the microglia hurts healthy brain cells by attacking the protein BA. The missing part of the argument concerns the acetylsalylicic acid.

We know the microglia attack the protein BA. (D) therefore suggests microglia might have some protective effect on cognitive functions, which doesn’t strengthen the argument that microglia cause deterioration of those functions.

It’s not clear what “immune reactions by microglia” are or how they relate to damage to cognitive faculties.