Dental Microbiology #211 IMMUNOLOGY 2006 Lecture 2 The Lymphocytes

Topics The Lymphocytes: B and T cells Antigen-specific Receptors on B and T cells CD4+ and CD8+ T cells Cytokines Antibody production: B cells and Plasma cells Immunological memory

The Lymphocyte Lymphocytes are mononuclear white blood cells that perform immunological functions and are constituents of the adaptive immune response Small Medium and Large (7 to 15 microns diameter) Two types of Lymphocytes: B (Bone marrow-derived) T (Thymus-derived) B and T cells have similar size and morphology but perform completely different functions

The Lymphocytes Fig 1

The Lymphocyte Fig 2

Antigen Specific Receptors Both B and T cells have on their surface cell- bound molecules that recognize and bind specific antigen. These molecules are called Antigen-specific receptors. B cell receptors (BCR), T cell receptors TCR. Each lymphocyte has many receptor molecules but all are identical on any given lymphocyte. Each lymphocyte will recognize and bind only one Ag specificity

Antigen specific Receptors Fig 3 Many lymphocytes, each expressing only one type of Ag- specific receptor When the lymphocyte encounters its specific Ag, the lymphocyte divides (proliferates) and gives rise to many more lymphocytes expressing the same Ag-specific receptor on their surface (clone)

Distinctions between B and T lymphocytes B cells start and end their development in the bone marrow T cell progenitors migrate from the bone marrow to the thymus where they develop as T cells under the influence of the thymic microenvironment. B cells synthesize antibodies. T cells do not produce antibodies but have other multiple functions

T cell types and functions There are two main classes of T cells: They are distinguished by the CD (cluster of differentiation) molecules they express on the surface: CD4 or CD8 CD4 T cells are called T helper (Th) cells CD8 T cells are called Cytotoxic T lymphocytes (CTL)

CD4+ and CD8+ T lymphocytes CD4 T (Th) cells: Help B cells to produce antibodies Participate in inflammatory reactions Enhance phagocytic activity of monocytes and macrophages CD8 T cells (CTL): Kill virus-infected cells and cells of foreign tissues and organ transplants by direct cell-to-cell contact

Clonal expansion: T cells Upon encounter with Ag T cells proliferate and become effector cells. Effector T cells produce powerful pharmacologic mediators called Cytokines. Cytokines affect the function of other cells as well as their own CD4 T cells produce Interleukins (IL-2, IL-4, IL-6, IL-10 etc), Interferon-gamma (IFN-g), Tumor Necrosis Factor, etc. CD8 T cells release cytotoxic molecules called perforins and granzymes which kill the target cells with which the CTL enters in close contact.

CTL/Target cell interactions Fig 4 b) Influenza-infected target cell (V) is killed by CTLs (T) T T T a) Viral particles on infected cell surface

CTL Fig 5

Clonal expansion: B cells As a result of contact with specific Ag, the B lymphocytes divide (proliferate) and give rise to many more B cells expressing the same Ag-specific receptor on their surface and producing the same specific soluble Ab. B cells synthesize Ab and differentiate into plasma cells Plasma cells synthesize and secrete Ab. Each B cell clone produces only one type of Ab specificity.

Antigen specific Receptors Fig 3 Efector cells produce Ab

The Adaptive immune response has Memory Encounter of a lymphocyte with its specific Ag gives rise to effector cells Effector cells have a limited life-span, and most cells die. The cells that persist after the Ag has disappeared are called memory cells and form the basis of immunological memory. A second encounter with the same Ag will result in a more rapid response  long-lasting protection.

Memory Contd. The Ab and T cell responses of an individual to the first encounter with a specific Ag is called primary response: slow and weak. The response to subsequent immunization with the same Ag (booster immunization), results in stronger and sustained Ab production and T cell responses called secondary or anamnestic response

The Secondary Ab response Fig 6

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