CELLS OF THE IMMUNE SYSTEM

The immune system consists of cells (leukocytes, dendritic cells, and macrophages) and different lymphoid organs (primary and secondary) and lymphoid tissues. Coordination between all the above three i.e., immune cells, lymphoid organs and lymphoid tissues is required for the development of a full immune response. Blood vessels and lymphatic systems connect these organs, tissues, and cells uniting them into a functional whole. Let us first learn about the cells of the immune system. 

Leukocytes The leukocytes are also called white blood corpuscles/cells or WBCs. They are mainly involved in immune responses, recognizing and neutralizing invaders such as bacteria, viruses, and cancerous cells. The total leukocyte count in blood of humans is 4000-11,000/mm3 .

Leukocytes

 The leukocytes are also called white blood corpuscles/cells or WBCs. They are mainly involved in immune responses, recognizing and neutralizing invaders such as bacteria, viruses, and cancerous cells. The total leukocyte count in blood of humans is 4000-11,000/mm3 . The leukocytes are subdivided, based on the presence of granules in their cytoplasm into the following categories.

a) Agranulocytes Agranulocytes as the name indicates are characterized by the absence of granules in their cytoplasm. There are two types of agranulocytes:

(i) Lymphocytes

(ii) Monocytes

 Granulocytes 

Granulocytes are white blood cells that are characterized by the presence of granules in their cytoplasm. They are also called polymorphonuclear leukocytes (PMN or PML) because of the varying shapes of the nucleus, which is usually lobed into three segments 

Granulocytes are further divided 

 (i) Neutrophils, 

(ii) Eosinophils and 

(iii) Basophils

They are characterized by stained granules within their cytoplasm under a microscope. Most of them contain neutrophilic granules (neutrophils), a few contain granules that stain with acidic dyes (eosinophils), and some have basophilic granules (basophils). The WBCs in the body acts as the first line of defense (especially neutrophils and monocytes) or provide immunity against invasion by microbes. These cells engulf (take inside) the invading organism through the process of phagocytosis and destroy them. Acting together, these cells provide the body with powerful defenses against viral, bacterial, and parasitic infections and cancer as well. Although all leukocytes have an important role in immunity, lymphocytes play a central role in immunity.