1. Dr.Mohammed Sharique Ahmed Quadri Assistant professor Physiology
بسم الله الرحمن الرحيم
T LYMPHOCYTES
Dr.Mohammed Sharique Ahmed Quadri
Assistant professor Physiology
Al Maarefa College

2. T (thymic) Lymphocytes
Lymphocytes migrate from bone marrow to the thymus for preprocessing to form “T” lymphocytes
Preprocessing in the thymus :
Cells divide rapidly - each thymic lymphocyte developing specific reactivity for one antigen
End result: thousands of T lymphocytes each with different specific reactivities for different antigens
Insuring that each T lymphocyte will not react with the body’s own antigens (self antigen)
Then the preprocessed cells leave thymus to lymphoid tissues
Most preprocessing of T lymphocytes occurs prior to and completely after birth

3. T Lymphocytes Carry out cell-mediated immunity
Clonal and antigen specific – acquire receptors in the thymus
T cells are activated for foreign attack only when it is on the surface of a cell that carries foreign and self antigens
Learn to recognize foreign antigens only in combination with a person’s own tissue antigens
A few days are required before T cells are activated to launch a cell-mediated attack

4. The T cell System
Exposure to specific antigen causes marked reproduction in specific T lymphocytes
Memory T cells are created (T-lymphocyte memory cells)
Mature T-cells have T cell receptors which have a very similar structure to antibodies and are specific to one antigen.
T cells respond to antigens only when they are bound to MHC proteins on the surface of antigen-presenting cells (macrophages, B lymphocytes, dendritic cells)

5. T Lymphocytes 2 main types of T cells
CD8 cells (cytotoxic, or killer T cells)
Destroy host cells harboring anything foreign
CD4 cells (mostly helper T cells)
Modulate activities of other immune cells
Secrete chemicals that amplify the activity of other immune cells
Β-cell growth factor
T-cell growth factor (interleukin 2)
Macrophage-migration inhibition factor
CD4+CD25+T cells / Suppressor T- cells( regulatory T cells)

6. Cytotoxic T Cells Direct attack (killer cells)
Secrete perforins (punch holes in cells)
Releases toxic substances directly into cells
Kills multiple cells
Important in destroying virus infected cells

7. Types of T Lymphocytes: Helper T cells
Most numerous
Form lymphokines (IL-2, 3, 4, 5, 6 and BCSF, BSDF)
Regulatory functions of lymphokines:
Stimulation of B cell growth and differentiation
Activation of the macrophage system
Positive feedback effect on the helper cells
They help in the functioning of Cytotoxic T – cells.
HIV virus destroys these cells & hence both the types of immunity are lost.

9. Suppressor T Cells
Capable of suppressing actions of cytotoxic and helper T cells
Prevent excessive damage to the body tissue – Immune tolerance
Known as regulatory T cells

10. Antigen Presentation
T-Lymphocytes respond only to antigens presented to them by antigen-presenting cells
Macrophages can be antigen-presenting cells
As macrophage engulfs and ingests microbe, it digests the microbe into antigenic peptides
Antigenic peptides bind to a MHC molecule which transports the bound antigen to the cell surface where it is presented to passing lymphocytes
Antigen-presenting macrophages secrete interleukin
Enhances differentiation and proliferation of now-activated T-cell clone

11. Self-antigens ( major histocompatibility complex/MHC)
Plasma membrane-bound glycoproteins called MHC molecules
Synthesis is directed by group of genes called major histocompatibility complex (MHC)
Exact pattern of MHC molecules varies from one individual to another ( BIOCHEMIAL FINGER PRINTS/ “MOLECULAR IDENTIFICATION CARDS).
FUNCTIONS:
- Directing response of T-lymphocytes
- Rejection of transplanted tissue

13. Immune System Tolerance of Self-Antigens
Tolerance refers to preventing the immune system from attacking the person’s own tissues
Mechanisms involved in tolerance
Clonal deletion
Clonal anergy
Receptor editing
Inhibition by regulatory T cells
Immunological ignorance
Immune privilege

14. Autoimmune Diseases Arise from loss of tolerance to self-antigens
e.g. multiple sclerosis, rheumatoid arthritis , myasthenia gravis
Causes :
Exposure of normally inaccessible self-antigens sometimes induces an immune attack against these antigens
Normal self-antigens may be modified by factors such as drugs, environmental chemicals, viruses, or genetic mutations so that they are no longer recognized and tolerated by the immune system.
Exposure of the immune system to a foreign antigen structurally identical to a self-antigen
May be related to pregnancy, arising from lingering fetal cells in the mother’s body after the pregnancy

15. Immune Diseases Due to abnormal functioning of the immune system
2 general ways
Immunodeficiency diseases
Too little immune response
Examples
severe combined immunodeficiency
AIDS
Inappropriate immune attacks
Too much or mistargeted immune response
Categories of inappropriate attacks
Autoimmune responses
Immune complex diseases
Allergies

16. Mechanisms of Immunity: A Summary
Recognition of an antigen as foreign – accomplished by macrophages and helper T-cells
Foreign antigen is phagocytized by a macrophage
Macrophage presents antigen material on its cell membrane
Helper T-cell is exposed to this part of the macrophage membrane and becomes sensitized
Once an antigen has been recognized, the activated helper T cells initiate one or both immune mechanisms.
Cell Mediated Immunity
Humoral Immunity

17. Β versus T Lymphocytes

18. References Human physiology by Lauralee Sherwood, seventh edition
Text book physiology by Guyton &Hall,11th edition
Text book of physiology by Linda .s contanzo,third edition