T LYMPHOCYTES Dr. Shaikh Mujeeb Ahmed Assistant professor Physiology Al Maarefa College
Mar 26, 2015
T LYMPHOCYTES
Dr. Shaikh Mujeeb Ahmed
Assistant professor Physiology
Al Maarefa College
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
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
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)
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)
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
Types of T Lymphocytes: Helper T cells
– Most numerous– Form lymphokines (IL-2, 3, 4, 5, 6)
– 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.
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
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
• 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
Self-antigens( major histocompatibility complex/MHC)
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: is a process by which B cells and T cells are
deactivated after they have expressed receptors for self-antigens and before they develop into fully immunocompetent lymphocytes
– Clonal anergy:lack of reaction by the body's defense mechanisms to foreign substances, and consists of a direct induction of peripheral lymphocyte tolerance.
– Receptor editing: occurs during the maturation of B cells is an attempt to change the specificity of the antigen receptor of self reactive immature B-cells
– Inhibition by regulatory T cells (T cells suppressor)
– Immunological ignorance: Anatomical barriers can separate the lymphocytes from the antigen e.g BBB
– Immune privilege:
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
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
Hypersensitivity
• When an immune reaction results in considerable damage to the body its called hypersensitivity.
• Four typesType I hypersensitivity (Anaphylaxis)Type II hypersensitivity (antibody mediated
cytotoxicity)Type III hypersensitivity (immune complex
disorder)Type IV hypersensitivity (delayed type of
hypersensitivity)
Type I hypersensitivity (Anaphylaxis)
• Mast cell degranulation• Ig E response• eg.
– Allergic rhinitis– Eczema– Acute urticaria
• Occurs within minutes• Mediators• Histamine• Slow reacting substance
of anaphylaxis (SRS-A)• Its called Atopy
Type II hypersensitivity (antibody mediated cytotoxicity)
• Immune reaction that damages antigen bearing cells
• Example – incompatible blood transfusion
Type III hypersensitivity (immune complex disorder)
• When antigen antibody complexes are deposited in normal tissues of the body where they fix complement.
• Complement activation damages the surrounding tissue cells.
• Damage of “innocent bystanders”
• eg. A form of glomerulonephritis
Type IV hypersensitivity (delayed type of hypersensitivity)
• Its mediated by macrophages that have been activated by T cells.
• Hypersensitivity starts after several hours and peak at 48 to 72 hrs.
• Characteristically associated with granuloma formation
• eg. Hypersensitivity to tuberculin which is present in M. tuberculosis
VACCINE
• Vaccine (vaccinus – pertaining to cows)
• By Edward Jenner for small pox.
• Act on the principle of “mock” infection
• Types of Vaccines
– Live, Attenuated Vaccines
– Inactivated Vaccines
– Subunit Vaccines
– Toxoid Vaccines
• Live, attenuated vaccines - Measles, mumps, rubella, polio
• Inactivated or killed vaccines- Cholera, flu, hepatitis A
• Toxoid vaccine - Diphtheria, tetanus
Mumps
النكاف
measles
الحصبة
Tetanus
الكزازDiphtheria
الخناق
Hepatitis
التهاب الكبد
Tuberculosis
مرض السل
Polioشلل األطفال
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
T helper
cell
TSuppressor
cell
Plasma cell
B cell
cytotoxic
T cellT
memory cell
Bmemory
cell
macrophage
Antigen
Processed Antigen
Lymphokines
IL2IL3IL4IL5IL6
Antibodies
(-)(-)
Β versus T Lymphocytes
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