Topics

• The B and T Lymphocytes• Antigen-specific Receptors on B and

T cells• CD4+ and CD8+ T cells• Cytokines• Antibody production• 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

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) cellsCD8 T cells are called Cytotoxic T lymphocytes (CTL)

CD4+ and CD8+ T lymphocytes

CD4 T (Th) cells: Help B cells to produce antibodiesParticipate in inflammatory reactionsEnhance 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: 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

Clonal expansion: T 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 cytokines called perforins and granzymes which kill the target cells with which the CTL enters in close contact.

CTL Fig 4

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 5

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