T cells, also known as T lymphocytes, are a type of white blood cell that plays a central role in cell-mediated immunity. There are several main types of T cells. Helper T cells (CD4+) activate and direct other immune cells and help stimulate B cell antibody production. Cytotoxic T cells (CD8+) directly kill infected cells and tumor cells. Regulatory T cells suppress immune responses and prevent autoimmunity. Memory T cells remember previous infections to mount faster responses. Natural killer T cells can kill infected and cancerous cells by releasing cytokines. T cells develop and mature in the thymus gland through positive and negative selection processes.

1. SRI PARAMAKALYANI COLLEGE
Reaccredited with A+ grade by NAAC with CGPA 3.39 in 3rd cycle
affiliated to Manonmaniam Sundaranar University
ALWARKURICHI-627412
Post graduate & Research Centre – Department of Microbiology (government aided)
II Sem core : IMMUNOLOGY
T CELLS AND ITS TYPES
SUBMITTED TO:
DR. S. VISHWANATHAN,
HEAD OF MICROBIOLOGY
DEPARTMENT.
SUBMITTED BY:
MAHESWARI.A
REG NO: 20221232516113
I M.Sc Microbiology

2. T-cell & Types

3. T - cells
• A T cell is a type of lymphocyte.
• T cells are one of the important white blood cells of
the immune system and plays a central role in
the adaptive immune response.
• It matures in the thymus and that brings about cell
mediated immunity.
• T lymphocytes are thymus dependent cells. They
mature under the influence of thymic hormones.
• They are highly in the blood and spleen

4. •During its maturation within the thymus, the T cell comes to
express a unique antigen-binding molecule, called the T-cell
receptor, on its membrane.
•T-cell receptors can recognize only antigen that is bound to cell-
membrane proteins called major histocompatibility complex
(MHC) molecules.

5. • When a naive T cell encounters antigen combined with a MHC
molecule on a cell, the T cell proliferates and differentiates into
memory T cells and various effector T cells.
• There are two well-defined subpopulations of T cells: T helper (TH)
and T cytotoxic (TC) cells. Although a third type of T cell, called a T
suppressor (TS) cell, has been identified.
• T helper and T cytotoxic cells can be distinguished from one another
by the presence of either CD4 or CD8 membrane glycoproteins on
their surfaces.
• T cells displaying CD4 generally function as TH cells, whereas those
displaying CD8 generally function as TC cells.

6. Functions of T cell
• Two main important functions of T cells are :
Effector : Cytolysis of cells infected with
microbes and tumor cells and lymphokine
production.
Regulatory : Either to increase or suppress
other lymphocytes and accessory cells

7. Scanning electron micrograph of a human T cell Scanning electron micrograph of a red
blood cell (left), a platelet (center), and
a T lymphocyte (right); colorized

8. T cells-Maturation
• The migration of progenitor T-cells from the early sites of hematopoiesis to
the thymus takes place at about day 11 of gestation in mice and in 8th or
9th week of gestation in humans.
• In thymus, the developing T cells are termed as thymocytes.
• These thymocytes proliferates and differentiates along developmental
pathways that produce functionally distinct sub-population of mature T-
cells.
• T-cells development initiates with the arrival of small numbers of lymphoid
precursors migrating from the blood into the thymus where they proliferate,
differentiate and undergo selection process that result in the development
of mature T cells.
• When T-cells precursor arrive at thymus, they don’t express the signature
surface markers of T cell as the T-cell receptors, the CD3 complex or the
co-receptors CD4 and CD8.

9. • The thymocytes early in the development lack detectable
CD4 and CD8.
• As these cells are CD4-, CD8-, they are termed as double
negative (DN) cells.
• The cells that enter the thymus DN1 cells are capable of giving
rise to all the subsets of T cells and are phenotypically C-kit+,
CD44 high and CD25-.
• The DN1 cells move from the corticomedullary junction into the
deeper cortex towards the subcapsular region.
• Once the DN1 cells encounter the thymic environment, they
begin to proliferate and express CD25 becoming C-kit+,
CD44 high and CD25+.
• These cells are called DN cells.

10. • The DN2 thymocytes then
experience a
rearrangement of genes
and the secretion of
cytokines like IL-7.
• The cells further
differentiate into the DN3.
• At the DN3 stage, the cells
mature into DN4, which is
further upregulated into
CD4 and CD8 cells
achieving a double
positive status in the
maturation process.

11. Thymic selection of T-cell:
Positive selection
• occurs in the cortical region of the
thymus.
• It involves the interaction of
immature thymocytes with cortical
epithelial cells.
• This interaction allows the
immature thymocytes to receive a
protective signal.
• This signal prevents them from
undergoing cell death.
• Cells whose receptors are not
able to bind MHC molecules would
not encounter with the thymic
epithelial cells and as a result it
would not receive the protective
signal resulting their death by
apoptosis.
Negative selection
• The cells that survive the positive
selection move into the medulla
and undergo negative selection,
which eliminates thymocytes with a
high affinity for self-antigens.
• The cells that interact too strongly
with the self-antigens receive an
apoptotic signal resulting in cell
death.
• During the same process,
however, some cells are selected
to form Treg cells. The cells that
successfully complete the selection
process exit the thymus as mature
naïve T cells.

13. T-cell activation:
• T-cells activation is initiated by interaction of the TCR-CD3 complex
with a processed antigenic peptide bound to either class-I (CD8+
cell) or class II (CD4+ cell) MHC molecules on the surface of an
antigen presenting cells.
• The initial signal is provided by the binding of the T cell receptor to
the cognate peptide present on the class II MHC. A similar peptide is
present on the class II MHC which activates the CD8+ cells.
• The second signal is provided as a result of co-stimulation, where
the surface receptors are induced by a relatively small number of
stimuli which are products of pathogens or breakdown products of
cells.
• After the two signals, T cells also receive stimulation in the form of
cytokines. The cytokine signal determines the fate of T cells,
especially in the case of helper T cells.

15. T-cell Differentiation
• The differentiation
of T cells into
different types of T
cells usually
occurs in the form
of lineage
commitment which
is based on the
affinity of the T-cell
receptor towards
self-antigen.

16. T Cell (T Lymphocyte) Types
• T cells can be grouped into categories depending
primarily on their function.
1.Helper T cells (CD4+ T cells)
2.Cytotoxic T cells (CD8+)
3.Regulatory T cells
4.T delayed hypersensitivity cells
4.Memory T cells
5.Natural killer T cells
c

17. 1. Helper T cells (CD4+)
• T helper cells are represented by Th.
• The T helper cells contain glycoprotein molecules called CD4
molecules on the surface. Hence T helper cells are called CD4₊
T cells.CD4₊ is an immunoglobulin.
• T helper cells recognizes the antigen-class II MHC complex of
antigen present cells
• Helper T cells play a central role in the functioning of the
adaptive immune system and are required for almost all
adaptive immune responses.

18. • Helper T cells: release interleukin-2 which in turn triggers the release of
other cytokines.
• Cytokines: any soluble factor secreted that act as a signal to other
lymphoid cells.
• There are two categories:-
1.Lymphokines: secreted by lymphocytes
2.Monokines: secreted by macrophages
Certain cytokines like interferon and interleukins are secreted by both
lymphocytes and macrophages

19. Activation of TH Cells
• Helper T cells become activated by interacting with antigen-
presenting cells, such as macrophages.
• Antigen-presenting cells ingest a microbe, partially degrade it,
and export fragments of the microbe—i.e., antigens—to the cell
surface, where they are presented in association with class
II MHC molecules.
• A receptor on the surface of the helper T cell then binds to the
MHC-antigen complex
• Helper T cell activation proceeds through stimulation by
a cytokine

21. • There are two different kinds of helper T cells.
• Th1 and Th2.
• Th1 functions to empower macrophages so that they can
destroy intravesicular pathogens (bacteria).
• Th2 cells function to activate B cells. Normally it is insufficient
for an antigen alone to stimulate the B cell to activation. The
Th2 cell when activated will bind to the B cell (CD40L and
CD40LR) and release cytokines Helper T cells

22. FUNCTIONS OF TH CELL
It activates macrophages to induce phagocytosis.
The cytokines of t cell produce allergy reactions.
It brings about humoral immune response by activating
B cells.
It brings about cell mediated immunity by activating T
cells.

23. 2.cytotoxic T cells
• Cytotoxic T cells are also known as ‘killer’ T cells ,
their role in the destruction of infected cells,
pathogens, and tumor cells.
• They are represented by Tc or Tk.
• Also known as CD8₊ T cells.
• The cytotoxic T cells can directly attack a cell by
killing microorganisms.
• They do this by the transfer of cytotoxic granules
to infected target cells, which kill the cell and any
pathogens it contains.

24. • cytotoxic granules containing proteins
called perforin and granzymes, which work together
to destroy target cells.
• When they encounter an infected cell, cytotoxic T
cells bind to the MHC I via their TCR receptors and
release their cytotoxic granules.
• Perforin creates holes in the cell membrane of the
target cell, and granzymes enter the cell via these
pores.
• Once inside, the granzymes
initiate apoptosis (programmed cell death) which kills
the cell and any pathogens it contains.

27. 3. Regulatory T cells
• Regulatory T cells (also called T Regs) are T cells which have a
role in regulating or suppressing other cells in the immune
system.
• Also called suppressor T cell, T Reg.
• There are two subsets of T reg cells. They are:
- t T Reg cells ( thymic T Reg cells)
- p T Reg cells ( pheripheral T Reg cells )
• Regulatory T (TReg) cells are essential for maintaining peripheral
tolerance, preventing autoimmune diseases and limiting chronic
inflammatory diseases.

29. 4.Delayed hypersensitivity T cells (TD)
• Brings macrophages to areas where delayed hypersensitivity
reaction occur.
• TD similar to TH cells.
• TD secrete primarily macrophage chemotoxin and macrophage
migration inhibition factor. By secreting these lymphokines the
TD cells are directly involved in the delayed hypersensitivity
reaction.
• Macrophage chemotoxin attracts monocytes and macrophages
whereas macrophage inhibition factor inhibits the migration of
macrophages and monocytes, thereby infiltration of these cells
occur into tissues where a delayed type hypersensitivity
reaction is occurring.

31. 4. Memory T cells
• Antigen-specific, long-lived memory T lymphocytes form following
an infection.
• Memory T lymphocytes are important because they can quickly
proliferate into large numbers of effector T lymphocyte upon re-
exposure to the antigen.
• They provide the immune system with memory against previously
encountered antigens.
• Memory T lymphocytes may either be CD4+ or CD8+.

32. • memory T cells are highly capable of
responding to antigens upon reintroduction–
which helps the body build immunological
tolerance.
• Memory cells are unique because they
remember pathogens and infectious cells
faster than others allowing them to fight off
bacteria and viruses quickly.
• Memory T cells are the reason vaccines can
create immunities in the body.

33. 5.NATURAL KILLER T CELLS
• Natural killer T cells are a type of T cell that also have certain
features of natural killer (NK) cells.
• They can kill invading microorganisms, such as bacteria and
viruses, by releasing cytokines.
• They can also kill certain cells, such as cancer cells, either
directly or indirectly by causing other immune cells to kill them.
• If natural killer T cells become inactive, this may contribute to
the development and progression of certain diseases, such as
cancer.

34. FUNCTIONS OF T CELLS
T cells bring about cell mediated immune response.
They kill bacteria and viruses hidden inside cells.
They kill infected cells.
They kill tumor cells.
Activate other cells such as B cells and macrophages.

35. References
• Kuby immunology, 7th edition
• https://www.onlinebiologynotes.com/t-lymphocyte-types-
functions/#:~:text=T%2Dcells%20originate%20in%20bone,and%
20recognize%20MHC%20bound%20antigen.
• https://www.onlinebiologynotes.com/t-cell-maturation-
activation-and-differentiation/
• https://microbenotes.com/t-cell-t-lymphocyte/

36. THANK YOU