**Question:** Tissue macrophages are called?

A. Dendritic cells
B. Langerhans cells
C. Monocytes
D. Microglia

**Core Concept:** Macrophages are a type of white blood cell that plays a crucial role in the immune system. They are involved in phagocytosis, which is the process of engulfing and breaking down pathogens and cellular debris. Macrophages are classified into two main categories:

**Why the Correct Answer is Right:**
Dendritic cells are a subset of macrophages that are specialized in capturing and presenting antigens to T-lymphocytes, initiating the adaptive immune response. They are primarily found in the lymph nodes and play a vital role in linking the innate and adaptive immune systems.

**Why Each Wrong Option is Incorrect:**
B. Langerhans cells are a specific type of dendritic cell, primarily found in the epidermis and involved in skin immunity. While they are a subset of dendritic cells, they are incorrectly listed as a separate entity in the choices.
C. Monocytes are a type of white blood cell that circulates in the blood and transmigrates into tissues, where they differentiate into macrophages. Monocytes are not tissue macrophages, but rather precursors to them.
A. Microglia are macrophages of the central nervous system (CNS), primarily involved in surveillance and immune defense within the brain and spinal cord. They are part of the resident immune cells of the CNS and are not relevant to tissue macrophages in other tissues.

**Clinical Pearl:**
Tissue macrophages are primarily distributed in various tissues throughout the body, including the liver, spleen, and bone marrow. They play a critical role in maintaining tissue homeostasis, phagocytosis, and immune defense against pathogens. Macrophages have a crucial role in the clearance of cell debris, apoptotic cells, and unhealthy cells, thus maintaining tissue integrity and preventing inflammation.

**Correct Answer:** D. Microglia are the tissue macrophages of the central nervous system (CNS). They contribute to immune defense within the brain and spinal cord, not in general tissue macrophages throughout the body.