This document discusses the different types of immune cells. It describes lymphocytes including B cells, T cells, and natural killer cells. It also discusses mononuclear phagocytes such as monocytes and macrophages. Granulocytic cells including neutrophils, eosinophils, and basophils are also covered. The document concludes by briefly mentioning mast cells, dendritic cells, and follicular dendritic cells.

1. Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET
Assistant Professor
Department of Microbiology
Sacred Heart College (Autonomous)
Tirupattur, Vellore District.
Mobile: 9994146964
E.mail: microsaranraj@gmail.com
CELLS OF IMMUNE
SYSTEM

2. LYMPHOID CELLS (or) LYMPHOCYTES
Lymphocytes constitute 20 % – 40 % of the body’s WBCs
and 99 % of the cells in the Lymph.
There are approximately 1011 Lymphocytes in the human
body.
Lymphocytes continually circulate in the blood and lymph
and are capable of migrating into the tissue spaces and
lymphoid organs.
Lymphoid cells are sub-divided into 3 categories. They are:
1)B – Lymphocytes
2)T – Lymphocytes
3)Natural Killer Cells (NK Cells)

3. B - Lymphocytes
B – Lymphocytes or B - cells mature in the Bone Marrow.
B - Lymphocytes differentiate into Plasma cells to secrete
Antibodies.
Once specific antigen binds to surface Immunoglobulin
molecule, the B - cells differentiate into Plasma cells that
produce and secrete these Antibodies.
Plasma cells are terminally differentiated cells, and many die
in 1 or 2 weeks.
The genesis of mature B - cells from pluripotent progenitor
stem cells occurs in the bone marrow and is Antigen
independent.
The activation of B - cells into antibody-producing and
antibody-secreting cells (Plasma cells) is Antigen dependent.

4. B - Lymphocytes
Mature B - cells can have 1 to 1.5 × 105 Immunoglobulin
receptors for Antigen.
If B - cells also interact with T - helper cells, they proliferate
and switch the Isotype (class) of immunoglobulin that is
produced while retaining the same Antigen - binding
specificity.
B – cell plays a major role in Humoral Immunity.

5. T - Lymphocytes
T - lymphocytes derive their name from their site of
maturation in the Thymus.
Like B - lymphocytes, T - cells have membrane receptors
for antigen.
T - lymphocytes are involved in the regulation of the
Immune response and Cell Mediated Immunity.
It also help B - cells to produce antibody.
T - lymphocytes are classified into 3 types. They are: (i) T –
Helper Lymphocytes (CD4 Molecules), (ii) Cytotoxic T –
Lymphocytes (CD8 Molecules) and (iii) Natural Killer T –
cells (NKTs).

6. T - Lymphocytes
(i) T – Helper Lymphocytes (CD4 Molecules) - Adhesion
molecule that binds to class II MHC molecules; Signal
transduction.
(ii) Cytotoxic T – Lymphocytes (CD8 Molecules) -
Adhesion molecule that binds to class I MHC molecules;
Signal transduction.
(iii) Natural Killer T – cells (NKTs) - NK T cells (NKTs)
are a heterogeneous group of T cells that share properties of
both T cells and NK cells. They recognize foreign lipids and
glycolipids and constitute only 0.2 % of all peripheral blood
T cells.

7. Natural Killer (NK) Cells
NK cells are large granular lymphocytes that kill tumor cells
and virus-infected targets.
NK cells constitute 5 % – 10 % of lymphocytes in human
peripheral blood.
NK cells does not have membrane receptors for antigen but
they have NK Cell receptors.
NK cells adhere to infected target cells and induce their cell
death via (i) Delivery of Apoptic signals (programmed cell
death in multicellular organisms) mediated by Perforins
(Pore forming cytolytic protein), Granzymes (Serine
protease released by cytoplasmic granules within Cytotoxic
T - Cells) & Tumor necrosis factor (Cell signaling proteins
involved in systemic inflammation; Type of Cytokines).

8. Natural Killer (NK) Cells
(ii) By Effector interactions via their surface Fas ligand
molecule (Transmembrane protein) with Fas on the target
cell.
NK cells can also kill through Antibody-dependent, Cell-
mediated cytotoxicity mechanisms via Cell surface receptors
for constant domains present on immunoglobulins.

9. MONONUCLEAR PHAGOCYTES
The Mononuclear phagocyte system (MPS) (or)
Reticuloendothelial system (or) Macrophage
system is a part of the immune system that consists of
the Phagocytic cells located in reticular connective
tissue.
The Mononuclear phagocytic system consists of two cells.
They are:
(i)Monocytes (Circulating in the Blood)
(ii)Macrophages (Present in the Tissues)
Monocytes and Macrophages are involved in Phagocytosis
(or) Intracellular killing of microorganisms.

10. Monocytes
Monocytes comprise up to 10 % of circulating WBCs.
Monocytes are motile and highly adherent upon
phagocytic activity.
During Hematopoiesis in the bone marrow, Granulocyte
-monocyte progenitor cells differentiate into
Promonocytes, which leave the bone marrow and
enter the blood, where they further differentiate into
mature Monocytes.
Monocytes circulate in the bloodstream for about 8
hrs, during which they enlarge; they then migrate into
the tissues and differentiate into Macrophages and
Dendritic cells (Antigen presenting cells).

12. Monocytes
Monocytes are involved in Phagocytosis and
Intracellular killing of microorganisms.
Monocytosis - State of excess monocytes in the
peripheral blood. It may be indicative of various disease
states.
Monocytopenia - Form of Leukopenia (decrease in
number of WBCs) associated with a deficiency of
Monocytes.

13. Macrophages
Macrophages are terminally differentiated, long - lived
Monocytes residing in reticular connective tissue.
Like Monocytes, Macrophages are motile and highly
adherent upon phagocytic activity.
Macrophages are first discovered by Elie Metchnikoff in
1884.
Macrophage-like cells serve different functions in different
tissues and are named according to their tissue location. (i)
Alveolar macrophages – Lungs, (ii) Histiocites – Connective
tissues, (iii) Kupffer cells – Liver, (iv) Mesangial cells –
Kidney, (v) Microglial cells – Brain and (vi) Osyeoclasts –
Bone.

15. Functions of Macrophages
Phagocytosis.
Adaptive immunity.
 Muscle regeneration.
Limb regeneration.
Wound healing.
Iron Homeostasis – Maintaining constant
concentrations of iron in the Brain internal
environment.

16. GRANULOCYTIC CELLS OR GRANULOCYTES
Granulocytes are a category of WBCs characterized by the
presence of granules in their cytoplasm.
Granulocytes are also called as Polymorphonuclear
leukocytes (PMN, PML, or PMNL).
Granulocytes are produced via Granulopoiesis in
the Bone marrow.
Granulopoiesis - Hematopoiesis of Granulocytes.
Based on Cellular morphology and Cytoplasmic staining
characteristics, Granulocytes are classified as
(i) Neutrophils
(ii) Eosinophils

17. Neutrophils
Neutrophils are the most highly abundant myeloid cell type,
comprising 40 % to 70 % of total WBCs.
The Neutrophils has a Multilobed nucleus and a
granulated cytoplasm that stains with both acid and basic
dyes.
Neutrophils are produced by hematopoiesis in the bone
marrow. They are released into the peripheral blood and
circulate for 7 – 10 hrs before migrating into the
tissues, where they have a life span of only a few days.
Neutrophils are Mobile and Phagocytic in nature.

18. Neutrophils ingest, kill, and digest microbial pathogens
with their functions dependent upon special proteins
found in Primary granules (Cationic defensins &
Myeloperoxidase) & Secondary granules (Iron
chelators, Lactoferrin & Digestive enzymes).
Neutrophilic granules contain multiple Antimicrobial
agents, including Oxygen independent Lysozyme
(Peptidoglycan degradation), Hydrogen peroxide and
Lactoferrin (Iron chelator).
Neutrophils dying at the site of infection contribute to
the formation of the whitish exudate called Pus.

20. Eosinophils
Eosinophils comprising about 1 % to 3 % of total WBCs.
The Eosinophil has a Bilobed nucleus and a granulated
cytoplasm that stains with the Acid dye Eosin red.
Eosinophils are produced by hematopoiesis in the bone
marrow and migrating into the tissues.
Eosinophils are Mobile and Phagocytic in nature.
Phagocytic role of Eosinophils is significantly less
important than that of Neutrophils.
Eosinophils play a major role in the defense against
parasitic organisms (by damaging the parasite
membrane) and Antibody mediated Cytotoxic
Hypersensitivity (Type – II Hypersensitivity).

22. Basophils
Basophils comprising less than 1 % of total WBCs.
Basophils has a Single lobed nucleus and heavily
granulated cytoplasm that stains with the Basic dye
Methylene blue.
Basophils are non – phagocytic in nature.
Basophils function by releasing pharmacologically
active substances from their cytoplasmic granules which
play a major role in certain Allergic responses.

24. MAST CELLS
Mast cells are formed in bone marrow by Hematopoiesis,
are released into the blood as undifferentiated cells; they do
not differentiate until they leave the blood and enter the
tissues.
Mast cells can be found in a wide variety of tissues,
including the skin, connective tissues of various organs, and
mucosal epithelial tissue of the respiratory, genitourinary
and digestive tracts. Like Basophils, these cells have large
numbers of Cytoplasmic granules that contain Histamine
and other pharmacologically active substances.
Mast cells, together with blood Basophils, play an important
role in the development of Allergic responses.

25. DENDRITIC CELLS (Langerhans Cell)
Dendritic cells are Antigen presenting cells.
The Dendritic cell acquired its name because it is covered
with long membrane extensions that resemble the dendrites
of nerve cells.
Four types of dendritic cells are known: (i) Langerhans
cells, (ii) Interstitial dendritic cells, (iii) Myeloid cells and
(iv) Lymphoid dendritic cells.
Dendritic cells arises from Hematopoietic stem cells via
different pathways and in different locations.
Dendritic cells acquire antigen by Phagocytosis.
Another type of Dendritic cell, the Follicular dendritic cell,
does not arise in bone marrow and has a different function
from the Antigen-presenting dendritic cells