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Although the correlation is not 100 percent, CD4-bearing T cells are associated with helper functions and CD8-bearing T cells are associated with cytotoxicity. These functional distinctions based on CD4 and CD8 markers are useful in defining the function of each type.

Helper t cells and their cytokines

Helper T cells (Th) , bearing the CD4 molecule, function by secreting cytokines that act to enhance other immune responses. There are two classes of Th cells, and they act on different components of the immune response. These cells are not distinguished by their surface molecules but by the characteristic set of cytokines they secrete ( [link] ).

Th1 cells    are a type of helper T cell that secretes cytokines that regulate the immunological activity and development of a variety of cells, including macrophages and other types of T cells.

Th2 cells    , on the other hand, are cytokine-secreting cells that act on B cells to drive their differentiation into plasma cells that make antibody. In fact, T cell help is required for antibody responses to most protein antigens, and these are called T cell-dependent antigens.

Cytotoxic t cells

Cytotoxic T cells (Tc) are T cells that kill target cells by inducing apoptosis using the same mechanism as NK cells. They either express Fas ligand, which binds to the fas molecule on the target cell, or act by using perforins and granzymes contained in their cytoplasmic granules. As was discussed earlier with NK cells, killing a virally infected cell before the virus can complete its replication cycle results in the production of no infectious particles. As more Tc cells are developed during an immune response, they overwhelm the ability of the virus to cause disease. In addition, each Tc cell can kill more than one target cell, making them especially effective. Tc cells are so important in the antiviral immune response that some speculate that this was the main reason the adaptive immune response evolved in the first place.

Regulatory t cells

Regulatory T cells (Treg) , or suppressor T cells, are the most recently discovered of the types listed here, so less is understood about them. In addition to CD4, they bear the molecules CD25 and FOXP3. Exactly how they function is still under investigation, but it is known that they suppress other T cell immune responses. This is an important feature of the immune response, because if clonal expansion during immune responses were allowed to continue uncontrolled, these responses could lead to autoimmune diseases and other medical issues.

Not only do T cells directly destroy pathogens, but they regulate nearly all other types of the adaptive immune response as well, as evidenced by the functions of the T cell types, their surface markers, the cells they work on, and the types of pathogens they work against (see [link] ).

Functions of T Cell Types and Their Cytokines
T cell Main target Function Pathogen Surface marker MHC Cytokines or mediators
Tc Infected cells Cytotoxicity Intracellular CD8 Class I Perforins, granzymes, and fas ligand
Th1 Macrophage Helper inducer Extracellular CD4 Class II Interferon-γ and TGF-β
Th2 B cell Helper inducer Extracellular CD4 Class II IL-4, IL-6, IL-10, and others
Treg Th cell Suppressor None CD4, CD25 ? TGF-β and IL-10

Chapter review

T cells recognize antigens with their antigen receptor, a complex of two protein chains on their surface. They do not recognize self-antigens, however, but only processed antigen presented on their surfaces in a binding groove of a major histocompatibility complex molecule. T cells develop in the thymus, where they learn to use self-MHC molecules to recognize only foreign antigens, thus making them tolerant to self-antigens. There are several functional types of T lymphocytes, the major ones being helper, regulatory, and cytotoxic T cells.

Questions & Answers

what are the layers of heart..?
Subhadatta Reply
epicardium , myocardium ,endocardium are three layers of heart
Antony
what is gestation
Antony
the period of development in mammals, from fertilisation of the ovum till birth
Paul
what's lactation?
Paul
secretion of milk from female breast
Antony
different between artery and veins?
Antony
ok
Paul
arteries caries oxygenated blood while veins caries deoxygenated blood except for pulmonary artery and pulmonary vein
Paul
can you guys HELP me with anatomy and physiology
Miriam
is this Mbbs first year course?
Zohaib
hi guys how are you today?
hingi Reply
fine and you
Bupe
good
Miriam
I would like to know that my self
Diana Reply
I need HELP
Miriam
can you guys help
Miriam
what do you need?
hingi
what help do you need
Onyeuku
and how should you be helped
Onyeuku
A _____ Propagation of the action potential. this is where the AP will travel down adjacent membranes on the neuron to transmit a message from one neuron to another. B. ______ K+ is the most prominent ion inside the cell and it is at resting membrane potential. C. _______ Repolarization occur;
Miriam
C. ______ Repolarization occur; allowing potassium ions to leak out of the cell changing the membrane permeability. D. ______ A stimulus changes the permeability of a patch of the membrane. this allow sodium ions to rush into the cell causing a change in the polarity of the membrane.
Miriam
E._____ the sodium- potassium pump pumps out sodium in potassium ions to restore the initial ionic conditions.
Miriam
is this course for first year Mbbs?
Zohaib Reply
yes...its for first year
Aftax
what does Mbbs means please?
Fatoumatta
what is clinical anatomy
Namweene Reply
study of clinical structures
Subhadatta
physiology of neurotransmission in ans
Pradip Reply
what about umbilical cord
Subhadatta Reply
nutrition passing tube
Antony
hi
Antony
it is a connection for transfer of hormones,blood,food,etc between child and mother during pregnancy.
Zohaib
yes..
Subhadatta
What the difference between anatomy and psychology
milika Reply
nothing
Sohil
hahaha Anatomy vs. Physiology. Students of anatomy learn about the structure of body parts, while those enrolled in physiologystudy the functions and relationships of body parts. While these two fields are often paired together in the same class or program title, they may also be offered separately.
Sohil
what is gross anatomy?
Vasco Reply
is the macroscopic form of anatomy where organs will be seen without the use of microscope
Kofi
yes
Aaliyah
yes
Memory
yes
godfrey
yes
Maddy
what is anatomy
Vasco Reply
is the structure of the part of the body
Kofi
when did we expect heat period
Kabiru Reply
when is pregnancy expected after ovulation
Kabiru
Minimum blood pressure
Hasnain Reply
120/80
AmirHameed
90/60
baila
120/80
Sri
were can anatomy work
capella Reply
in hospitality
Sri
what is normal flora
Cris Reply
it's one kind of bacteria
Tariq
the smallest blood vessel is capillary
Richard Reply
How does deoxygenated blood become oxygenated during blood circulation?
Richard
in lungs through alveoli ...
Subhadatta

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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