• WBCs are the principle cells of immune system formed hematopoietic stem cell by the process of hematopoiesis. Hematopoiesis occurs in yolk sac during 1^st week of gestation. After 3^rd month of gestation, hematopoiesis occurs in liver and spleen of fetus and after birth, it occurs in bone marrow.

The cells of immune system are:

1. Lymphocytes-

• T-lymphocytes
• B- lymphocytes
• NK cell

2. Phagocytic cells

• Monocytes
• Macrophages

3. Granulocytic cells

• Neutrophils
• Basophils
• Eosinophils

4. Dendritic cells

I. Lymphocytes:

• Lymphocytes are small, round cells found in peripheral blood, lymph, lymph nodes, lymphoid organs and in tissues.
• Lymphocytes represent 20-45% of total cells in peripheral blood and 99% of total cells in lymph and lymph node.
• According to side lymphocytes are divided into small (5-8µm), medium (8-12µm) and large (12-15µm).
• Depending on life span lymphocytes are classified into short lived (2 weeks) and long lived (3 years or more or even lifelong).

Broadly lymphocytes are divided into three sub-populations, on the basis of function and cell membrane components.

1. T-lymphocytes
2. B-lymphocytes
3. Natural killer cell

II. Phagocytic cells:

• Monocytes and macrophages are mononuclear phagocytic cells.
• Granulocyte-monocyte progenitor cell differentiates into promonocytes and neutrophil.
• Promonocytes leaves the bone marrow and enter into blood stream where they differentiate into mature monocytes.
• Monocytes circulates in blood for about 8 hours, during which they enlarges and then enter into tissues and differentiates into specific macrophages and dendritic cells.

1. Monocytes:

• Blood monocytes measure 12-15 µm with a single lobed kidney shaped nucleus.
• It accounts for (2-8%) of blood leucocytes.

Immunological Functions of monocytes:

• Helps in antigen processing and presentation
• Releases cytokines
• Specialized function in tissues
• Cytotoxicity

2. Macrophages:

• Monocyte migrates to tissue and differentiates into macrophages.
• Differentiation of monocytes into macrophages involves following changes:
• Cell enlarges 5-10 folds
• Intracellular granules increases in number and complexity
• Increase phagocytic ability
• Produces higher level of hydrolytic enzymes and cytokines
• Macrophages serve different functions in different tissues.
• Alveolar macrophages : in lungs
• Histiocyte: connective tissue
• Kuffer cell: liver
• Messangial cell: kidney
• Microglial cell: brain
• Osteoclast: bone

Immunological functions of macrophages:

• Phagocytosis
• Antigen presentation to T-cell
• Secretion of lymphokines IL-1, IL-6. IL-12. TNF-α etc to activates inflammatory response
• Secretion of granulocyte monocyte colony (GMC) stimulating factors.

II. Granulocytic cells:

1. Neutrophil:

• Neutrophils are (11-14µm) in diameter with multilobed nucleus with granules in cytoplasm.
• It constitutes 50-70 % of total circulating WBC and remains for 7-8 hours in blood and then migrates to tissues
• Life span is 3-4 days.
• Also known as polymorphonuclear (PMN) leucocyte.
• Neutrophils is stained by both acidic and basic dye.

Immunological functions of Neutrophil:

• Phagocytic role in acute inflammatory response.
• It is the first immune cell to responds in inflammation.

2. Eosinophils:

• Eosinophils are (11-15µm) in diameter, heavily granulated with bilobed nucleus
• It is stained by acidic dye ie Eosin
• They are phagocytic and motile

Immunological functions of Eosinophil:

• Granules contain various hydrolytic enzymes that kill parasites which are too large to be phagocytosed by neutrophils.
• Provide allergic inflammation

3. Basophils:

• Basophils are non-phagocytic cell found in small number in blood and tissue
• Cytoplasm contains large number of prominent basophilic granules containing histamine, heparin, serotonin, and other hydrolytic enzymes
• Stained by basic dyes

Immunological functions:

• Provide anaphylactic and atopic allergic reaction

IV. Dendritic cell:

• Dendritic cells have long cytoplasmic externsions that resembles to dendrites of nerve cell.
• They have highly pleomorphic with a small central body and many long needle like processes.
• Dendritic cells are antigen presenting cell (APC) because they possess MHC class.

Immunological functions:

• Involved in antigen presentation to T-cells during primary immune response.
• Very little role in phagocytosis.

Cells of immune system: Lymphocytes, phagocytic cell, granulocytes and dendritic cells

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