**Core Concept**
The major basic protein (MBP) is a key constituent of eosinophils, which are a type of white blood cell involved in the body's defense against parasites and in allergic reactions. MBP plays a crucial role in the eosinophil's ability to kill and digest foreign particles.

**Why the Correct Answer is Right**
Eosinophils contain a significant amount of MBP, which is stored in granules within the cell. When an eosinophil encounters a foreign substance or allergen, it releases MBP, which then interacts with the target cell to cause damage and destruction. MBP's basic nature allows it to bind to negatively charged molecules, such as those found on the surface of parasites or allergens, making it an effective tool for eosinophils to target and eliminate these substances. This process is an essential part of the eosinophil's function in fighting infections and reducing inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Neutrophils, another type of white blood cell, contain different types of granules, including azurophilic and specific granules, but not major basic protein.
**Option C:** Basophils, the least abundant type of white blood cell, are involved in inflammation and allergic reactions, but they do not contain major basic protein.
**Option D:** Lymphocytes, which include B cells and T cells, are a type of white blood cell that plays a crucial role in the immune system, but they do not contain major basic protein.

**Clinical Pearl / High-Yield Fact**
It's essential for clinicians to recognize the role of eosinophils in allergic reactions and parasitic infections, as well as the importance of MBP in eosinophil function. This knowledge can inform the development of targeted treatments for conditions such as asthma and parasitic infections.

**✓ Correct Answer: B. Eosinophil**