Lymphocytes and B Lymphocytes Overview

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Questions and Answers

What percentage of the recirculating pool of small lymphocytes do B cells form?

20%

15% (correct)

10%

Which of the following does not represent a subtype of B lymphocytes?

Memory B cells

Regulatory B cells (correct)

Immature B cells

Pre-B cells

Which surface molecules do mature B cells express?

IgA and IgE

CD3 and CD8

IgM and IgD (correct)

IgG and IgM

Which organ is primarily involved in the maturation of B lymphocytes?

Bone marrow Signup and view all the answers

What is the primary function of B cells after antigen stimulation?

To synthesize and secrete antibodies Signup and view all the answers

Which cluster of differentiation (CD) molecule is expressed on immature B cells?

CD19 Signup and view all the answers

Where do mature B cells primarily await the arrival of specific antigens?

Germinal centers of lymph nodes Signup and view all the answers

What type of immunity are lymphocytes primarily associated with?

Adaptive immunity Signup and view all the answers

What is the origin of T lymphocytes?

From lymphoid stem cells in bone marrow Signup and view all the answers

Which type of cells mature T lymphocytes in the thymus?

Thymic epithelial cells Signup and view all the answers

What is true about double-negative immature thymocytes?

They do not express CD4 or CD8 molecules Signup and view all the answers

What component of the TCR is responsible for recognizing specific peptide antigens?

Variable region of TCR Signup and view all the answers

What function does CD3 serve in T cells?

To transmit intracellular activation signals Signup and view all the answers

How do T lymphocytes recognize antigens?

By recognizing processed protein antigens in association with MHC molecules Signup and view all the answers

What distinguishes different clones of T lymphocytes?

The specific peptide antigens they can recognize Signup and view all the answers

Which of the following statements about T lymphocyte maturation is incorrect?

All immature thymocytes are found in the medulla. Signup and view all the answers

Which receptor is NOT associated with natural killer cells?

CD4 Signup and view all the answers

What percentage of peripheral lymphocytes do natural killer cells comprise?

5-10% Signup and view all the answers

Which of the following is a main function of natural killer cells?

Cytotoxic activity against tumor cells Signup and view all the answers

What role do LAK cells serve in cancer therapy?

They enhance the cytotoxic effect of NK cells. Signup and view all the answers

In immunotherapy, which lymphocyte subtype is known for its spontaneous non-specific killing of tumor cells?

Natural killer cells Signup and view all the answers

What happens to T cells that recognize MHC class I on thymic epithelial cells during maturation?

They preserve CD8 and lose CD4 expression. Signup and view all the answers

What is the main function of T helper cells (CD4) within the immune response?

Orchestrate the immune response through cytokine secretion. Signup and view all the answers

Which T cell subtype is primarily responsible for terminating the immune response?

T regulatory (Treg) cells Signup and view all the answers

What is the typical ratio of CD4 to CD8 T cells in most tissues?

2:1 Signup and view all the answers

During negative selection, what normally happens to self-reactive T cell clones?

They are deleted by apoptosis. Signup and view all the answers

Which T cell subtype is responsible for the fast and strong secondary immune response upon re-exposure to an antigen?

T memory cells Signup and view all the answers

What is a characteristic function of T cytotoxic (CD8) cells?

They kill cells infected with intracellular organisms. Signup and view all the answers

Which statement accurately describes the life span and location of T helper cells?

Relatively long-lived and predominating in the thymic medulla. Signup and view all the answers

Study Notes

Lymphocytes

Lymphocytes are a type of white blood cell and the unique cells of adaptive immunity.
CD molecules (Cluster of Differentiation):
- Cell surface molecules expressed on all immune cells
- Each type of immune cell has a particular CD number
- Examples:
  - T cells: CD3
  - Th cells: CD4
  - Tc cells: CD8
  - B cells: CD19

B Lymphocytes

Origin and Maturation:
- Develop from bone marrow stem cells
- Mature in the generative lymphoid organs (bone marrow)
- Circulate through the blood to complete maturation in secondary lymphoid organs
Subtypes:
- Pre-B cells: Lack surface immunoglobulins but express IgM heavy chains mu in their cytoplasm
- Immature B cells: Express IgM receptors on the surface
- Mature B cells: Express IgM and IgD molecules on the surface as receptors for antigens
- **Memory B cells: ** Express IgG, IgA, or IgE on the surface
Percentage, Life Span, and Location:
- B cells constitute about 15% of the recirculating pool of small lymphocytes
- Short life span (days or weeks)
- Found in the germinal center of lymph nodes, the white pulp of the spleen, and the gut-associated lymphoid tissue
Function:
- Important antigen-presenting cells
- Mature B cells differentiate to plasma cells which synthesize and secrete antibodies.

T Lymphocytes

Origin:
- From lymphoid stem cells in bone marrow
- Immature thymocytes circulate in blood without surface markers
Maturation:
- Develop from bone marrow stem cells
- Mature in the thymus
- Circulate through blood to secondary lymphoid organs.
- Naive lymphocytes respond to foreign antigens in these secondary tissues or return to the blood via lymphatic drainage
T lymphocyte receptor (TCR):
- Recognizes only processed protein antigens (epitopes)
- Recognizes epitopes in association with MHC molecules
- MHC molecules are found on the surface of antigen-presenting cells (APCs)
Maturation in the Thymus:
- The thymus consists of an outer cortex and an inner medulla
- Contains thymic epithelial cells, dendritic cells, and macrophages
Stages of Maturation:
- Double-negative immature thymocytes (in cortex): Don't express CD4 or CD8 molecules
- Double-positive immature thymocytes (in cortex): Express TCR complex, CD4, and CD8
- Expression of TCR/CD3 complex:
  - TCR: Composed of alpha and beta polypeptide chains with constant and variable regions
  - Variable region: Recognizes specific peptide antigens (epitopes) on MHC molecules of APCs
  - CD3: Associated with the TCR and transmits intracellular activation signals
- Single-positive mature T lymphocytes: Express either CD4 or CD8 based on TCR recognition of MHC class I or II
  - CD8 + cells: Become T cytotoxic cells
  - CD4 + cells: Become T helper cells
- Negative selection: Ensures self-tolerance by deleting T cell clones that recognize self-peptides (self-reactive clones)
Subtypes:
- T cytotoxic cells (Tc) or killer cells: Kill cells infected with intracellular organisms or abnormal host cells (e.g., graft cells and tumor cells) by releasing cytotoxic proteins like perforins and granzymes
- T regulatory cells (Treg): Terminate the immune response by suppressing the functions of activated immune cells, express CD25
- T helper cells (Th): Orchestrate immune responses through cytokine secretion, resulting in the activation, proliferation, and differentiation of B cells, T cells, and macrophages
- Th1 and Th2: Subtypes of T helper cells
- T memory cells: Recognize antigens upon re-exposure, resulting in a faster and stronger secondary immune response
Percentage, Life Span, and Location:
- T cells constitute 65-80% of circulating lymphocytes (Th cells 65%, Tc cells 35%)
- Relatively long life span (months or years)
- T helper cells predominate in the thymic medulla, tonsils, and blood.

Natural Killer Cells (NK cells)

Origin: Lymphoid stem cells in the bone marrow
Receptors: CD56 and CD16 (receptor for the Fc portion of IgG)
Percentage: 5-10% of peripheral lymphocytes
Function:
- Innate immunity: Have spontaneous non-specific cytotoxic activity on virus-infected cells, tumor cells, and graft cells.
- Cytokine production: Produce INF-γ, TNF α, and GM-CSF (Granulocytes monocytes colony stimulating factor)
- LAK cells: IL-2-activated NK cells with more efficient killing effects than NK cells alone, showing promising results in metastatic cancer therapy
- ADCC (Antibody-dependent cellular cytotoxicity): Through the Fc receptor of IgG, NK cells can interact with and kill IgG-coated cells (Acquired immunity)

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Description

This quiz explores the essential characteristics of lymphocytes, focusing particularly on B lymphocytes. It covers their origin and maturation, subtypes, and the surface molecules that define their function within the immune system. Test your knowledge of these critical components of adaptive immunity.