9.3.2 Role of T Cells in Third Line of Defence (cell Mediated Immune Response)

T cells originate from stem cells in the bone marrow. After early embryonic development, the newly forming T cells migrate to thymus gland for processing. The thymus makes T cells immunocompetent that is capable of immunological response. The immune response provided by T cells is called cell mediated immune response.

Definition

T Cells: T cells originate from stem cells in the bone marrow. After early embryonic development, the newly forming T cells migrate to thymus gland for processing. The thymus makes T cells immunocompetent that is capable of immunological response.

Cell-Mediated Immune Response: The immune response provided by T cells is called cell mediated immune response.

Main Idea

• T cells originate from stem cells in the bone marrow.
• After early embryonic development, they migrate to thymus gland for processing.
• The thymus makes T cells immunocompetent.
• Immunocompetent means capable of immunological response.
• The immune response provided by T cells is called cell mediated immune response.

Activation of T Cells

• In case of infection, macrophages perform phagocytosis of invaded microorganism and detect particular antigens (nonself molecules) of the organism.
• Macrophages isolate and display these antigens on their surface with the help of their own proteins (self-protein) called major histocompatibility complex (MHC).
• In this way, macrophages become antigen-presenting cells (APCs).
• Macrophages also secrete interleukin-1 that stimulates and attracts helper T cells towards the displayed antigen.
• Helper T cells have specific receptor on their surface called T cell receptor (TCR) by which they bind with a particular antigen displayed on APC.
• Interleukin-1 also stimulates the helper T cells to secrete another protein, the interleukin-2 that not only compel the helper T cells to divide but also causes the proliferation of certain cytotoxic T cells and B cells.
• The activation of T-cells by a specific antigen is called cell-mediated immunity.
• The body contains millions of different T-cells, each able to respond to one specific antigen.

Types of T Cells

• The first group, known as CD8 cells because they have surface marker designated CD8, include cytotoxic T cells and suppressor T cells.
• The second group i.e., helper T cells also known as CD4 cells because they have a surface marker designated CD4.

When T cells are activated, they divide and produce four types of cells which have specific role in cell-mediated immune response.

• Cytotoxic T cells — These cells secrete cytotoxic which triggers destruction of the pathogen's DNA or perforin which is a protein that creates holes in the pathogens plasma membrane. The holes cause the pathogen to lyse (rupture).
• Helper T cells — These cells secrete interleukin 2 which stimulates cell division of T cells and B cells. In other words, these cells recruit even more cells to help fight the pathogen.
• Suppressor T cells — When infection is successfully removed, these cells begin to secrete certain proteins that inhibit further proliferation of T cell. Therefore, they shut down the immune response.
• Memory T cells — These cells remain dormant after the initial exposure to an antigen. If the same antigen presents itself again, even if it is years later, the memory cells are stimulated to convert themselves into helper T cells and help fight the pathogen.

Core Concept

• T cells = originate from stem cells in bone marrow
• Thymus = site of T cell processing
• Immunocompetent = capable of immunological response
• Cell-mediated immune response = provided by T cells
• Antigen-presenting cells (APCs) = macrophages displaying antigens
• MHC = major histocompatibility complex
• **TCR** = T cell receptor
• Interleukin-1 = secreted by macrophages
• Interleukin-2 = secreted by helper T cells
• Types of T cells = cytotoxic, helper, suppressor, memory
• CD8 cells = cytotoxic and suppressor T cells
• CD4 cells = helper T cells

Detailed Explanation

Role of T Cells in Third Line of Defence (Cell Mediated Immune Response)

• T cells originate from stem cells in the bone marrow.
• After early embryonic development, the newly forming T cells migrate to thymus gland for processing.
• The thymus makes T cells immunocompetent that is capable of immunological response.
• The immune response provided by T cells is called cell mediated immune response.

Activations of T Cells

• In case of infection, macrophages perform phagocytosis of invaded microorganism and detect particular antigens (nonself molecules) of the organism.
• Macrophages isolate and display these antigens on their surface with the help of their own proteins (self-protein) called major histocompatibility complex (MHC).
• In this way, macrophages become antigen-presenting cells (APCs).
• Macrophages also secrete interleukin-1 that stimulates and attracts helper T cells towards the displayed antigen.
• Helper T cells have specific receptor on their surface called T cell receptor (TCR) by which they bind with a particular antigen displayed on APC.
• Interleukin-1 also stimulates the helper T cells to secrete another protein, the interleukin-2 that not only compel the helper T cells to divide but also causes the proliferation of certain cytotoxic T cells and B cells.
• The activation of T-cells by a specific antigen is called cell-mediated immunity.
• The body contains millions of different T-cells, each able to respond to one specific antigen.

Types of T Cells

Two main categories of T cells have been identified. The first group, known as CD8 cells because they have surface marker designated CD8, include cytotoxic T cells and suppressor T cells. The second group i.e., helper T cells also known as CD4 cells because they have a surface marker designated CD4.

When T cells are activated, they divide and produce four types of cells which have specific role in cell-mediated immune response.

(i) Cytotoxic T Cells

• These cells secrete cytotoxic which triggers destruction of the pathogen's DNA or perforin which is a protein that creates holes in the pathogens plasma membrane.
• The holes cause the pathogen to lyse (rupture).

(ii) Helper T Cells

• These cells secrete interleukin 2 which stimulates cell division of T cells and B cells.
• In other words, these cells recruit even more cells to help fight the pathogen.

(iii) Suppressor T Cells

• When infection is successfully removed, these cells begin to secrete certain proteins that inhibit further proliferation of T cell.
• Therefore, they shut down the immune response.

(iv) Memory T Cells

• These cells remain dormant after the initial exposure to an antigen.
• If the same antigen presents itself again, even if it is years later, the memory cells are stimulated to convert themselves into helper T cells and help fight the pathogen.

Summary Table of T Cells

Summary of Interleukins

Intermediate Level

Role of T Cells in Third Line of Defence

Types of T Cells

Activation of T Cells

Interleukins

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. T cells originate from stem cells in bone marrow.
• 2. They migrate to thymus for processing.
• 3. Thymus makes T cells immunocompetent.
• 4. Cell-mediated immune response is provided by T cells.
• 5. Macrophages display antigens using MHC.
• 6. Macrophages become antigen-presenting cells.
• 7. Macrophages secrete interleukin-1.
• 8. Interleukin-1 attracts helper T cells.
• 9. Helper T cells bind to antigen via TCR.
• 10. Helper T cells secrete interleukin-2.
• 11. Interleukin-2 stimulates cell division.
• 12. Four types of T cells: cytotoxic, helper, suppressor, memory.
• 13. Cytotoxic T cells destroy pathogens.
• 14. Helper T cells recruit more cells.
• 15. Suppressor T cells shut down immune response.
• 16. Memory T cells remember antigens.
• 17. CD8 cells include cytotoxic and suppressor T cells.
• 18. CD4 cells are helper T cells.
• 19. Body contains millions of different T cells.
• 20. Each T cell responds to one specific antigen.

Remember

• - T cells = originate from stem cells in bone marrow
• - Thymus = site of T cell processing
• - Immunocompetent = capable of immunological response
• - Cell-mediated immune response = provided by T cells
• - Antigen-presenting cells (APCs) = macrophages displaying antigens
• - MHC = major histocompatibility complex
• - **TCR** = T cell receptor
• - Interleukin-1 = secreted by macrophages
• - Interleukin-2 = secreted by helper T cells
• - Types of T cells = cytotoxic, helper, suppressor, memory
• - CD8 cells = cytotoxic and suppressor T cells
• - CD4 cells = helper T cells

Common Mistakes

• 1. Confusing T cells and B cells — T cells = cell-mediated; B cells = humoral.
• 2. Mixing up the types of T cells — Cytotoxic, helper, suppressor, memory.
• 3. Forgetting the role of the thymus — Site of T cell processing.
• 4. Mixing up the interleukins — IL-1 from macrophages; IL-2 from helper T cells.
• 5. Forgetting the role of MHC — Displays antigens on surface.
• 6. Mixing up CD4 and CD8 — CD4 = helper T cells; CD8 = cytotoxic and suppressor T cells.
• 7. Believing all T cells are the same — Four types with different functions.
• 8. Forgetting the role of memory T cells — Provide long-lasting immunity.
• 9. Mixing up the functions — Cytotoxic: destroy; helper: recruit; suppressor: shut down.
• 10. Thinking T cells are not important — They are essential for cell-mediated immunity.

Quick Revision

T cells originate from stem cells in the bone marrow. After early embryonic development, the newly forming T cells migrate to thymus gland for processing. The thymus makes T cells immunocompetent that is capable of immunological response. The immune response provided by T cells is called cell mediated immune response. In case of infection, macrophages perform phagocytosis of invaded microorganism and detect particular antigens (nonself molecules) of the organism. Macrophages isolate and display these antigens on their surface with the help of their own proteins (self-protein) called major histocompatibility complex (MHC). In this way, macrophages become antigen-presenting cells (APCs). Macrophages also secrete interleukin-1 that stimulates and attracts helper T cells towards the displayed antigen. Helper T cells have specific receptor on their surface called T cell receptor (TCR) by which they bind with a particular antigen displayed on APC. Interleukin-1 also stimulates the helper T cells to secrete another protein, the interleukin-2 that not only compel the helper T cells to divide but also causes the proliferation of certain cytotoxic T cells and B cells. The activation of T-cells by a specific antigen is called cell-mediated immunity. The body contains millions of different T-cells, each able to respond to one specific antigen. Two main categories of T cells have been identified. The first group, known as CD8 cells because they have surface marker designated CD8, include cytotoxic T cells and suppressor T cells. The second group i.e., helper T cells also known as CD4 cells because they have a surface marker designated CD4. When T cells are activated, they divide and produce four types of cells which have specific role in cell-mediated immune response. Cytotoxic T cells secrete cytotoxic which triggers destruction of the pathogen's DNA or perforin which is a protein that creates holes in the pathogens plasma membrane. Helper T cells secrete interleukin 2 which stimulates cell division of T cells and B cells. Suppressor T cells begin to secrete certain proteins that inhibit further proliferation of T cell. Memory T cells remain dormant after the initial exposure to an antigen.

60-Second Revision

• T cells = originate from stem cells in bone marrow
• Thymus = site of T cell processing
• Immunocompetent = capable of immunological response
• Cell-mediated immune response = provided by T cells
• Antigen-presenting cells (APCs) = macrophages displaying antigens
• MHC = major histocompatibility complex
• TCR = T cell receptor
• Interleukin-1 = secreted by macrophages
• Interleukin-2 = secreted by helper T cells
• Types of T cells = cytotoxic, helper, suppressor, memory
• CD8 cells = cytotoxic and suppressor T cells
• CD4 cells = helper T cells
• Warning: Helper T cells are CD4; cytotoxic and suppressor T cells are CD8

NEXT TOPIC

• Chapter:
• 9. Immunity

• Topic:
• 9.3.3 ROLE OF B CELLS IN THIRD LINE OF DEFENCE (HUMORAL/ANTIBODY MEDIATED IMMUNE RESPONSE)

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