**Lymphocytes: The Smallest Type of Leukocytes in a Healthy Adult Man**

In a healthy adult man, the smallest type of leukocytes are lymphocytes. Lymphocytes are a type of white blood cell that plays a crucial role in the immune system. They are responsible for recognizing and destroying foreign substances, such as bacteria and viruses, as well as cancer cells.

**Explanation:**

1. **Leukocytes:** Leukocytes, also known as white blood cells, are an important component of the immune system. They are responsible for protecting the body against infections and foreign invaders.

2. **Types of Leukocytes:** There are several types of leukocytes, including neutrophils, basophils, eosinophils, monocytes, and lymphocytes. Each type of leukocyte has its own specific functions and characteristics.

3. **Basophils:** Basophils are a type of leukocyte that is involved in allergic reactions and the immune response to parasites. They are slightly larger than lymphocytes and have granules in their cytoplasm.

4. **Monocytes:** Monocytes are another type of leukocyte that are involved in the immune response. They are larger than lymphocytes and have a kidney-shaped nucleus. Monocytes can differentiate into macrophages, which are responsible for engulfing and destroying pathogens.

5. **Eosinophils:** Eosinophils are a type of leukocyte that are involved in allergic reactions and the immune response to parasites. They are slightly larger than lymphocytes and have granules in their cytoplasm. Eosinophils are particularly effective against parasitic infections.

6. **Lymphocytes:** Lymphocytes are the smallest type of leukocytes in a healthy adult man. They have a large nucleus and a small amount of cytoplasm. Lymphocytes are divided into three main subtypes: T cells, B cells, and natural killer (NK) cells. They play a crucial role in the immune response, including the recognition and destruction of foreign substances.

7. **Role of Lymphocytes:** Lymphocytes are involved in both the innate and adaptive immune responses. They are responsible for recognizing specific antigens and initiating an immune response against them. T cells are involved in cell-mediated immunity, B cells produce antibodies, and NK cells are responsible for killing infected or cancerous cells.

In conclusion, lymphocytes are the smallest type of leukocytes in a healthy adult man. They play a crucial role in the immune response by recognizing and destroying foreign substances.