6.5

Specific Immunity and T Cells

• TMB lymphocytes play a major role in the specific immune response.

• Understanding MHCs (Major Histocompatibility Complexes) is crucial for grasping this immune mechanism.

Major Histocompatibility Complex (MHC)

• MHCs are akin to bulletin boards displaying antigens to the environment.

• Two types of MHC:

  • MHC I: Expressed in all nucleated cells.

    • Allows communication with cytotoxic T cells (CD8+ T cells).

  • MHC II: Found only in immune system cells (macrophages, dendritic cells, B cells).

    • These are known as professional antigen presenting cells (APCs).

• Nonprofessional antigen presenting cells express MHC I, whereas professional APCs express MHC II.

Immune Response

• Each T or B lymphocyte can recognize specific antigens based on genetic determination.

• The human body can recognize billions of different antigens and mount immune responses effectively.

• The immune system avoids responding against self-antigens (self-tolerance).

• T lymphocytes coordinate cell-mediated immunity, while B lymphocytes coordinate antibody-mediated immunity.

Functions of T Lymphocytes

• T lymphocytes primarily have two functions:

  • Destroy infected or damaged cells.

  • Support other immune cells.

Epitopes

• T cells recognize specific parts of antigens, known as epitopes.

  • Various parts of an antigen (e.g., a component of a protein from bacteria) can be epitopes.

• Each type of T cell responds to unique epitopes through their receptors.

Types of T Cells

• Cytotoxic T Cells (CD8+):

  • Recognize antigens presented on MHC I and destroy infected cells.

• Helper T Cells (CD4+):

  • Recognize antigens presented on MHC II and enhance the immune response by releasing cytokines.

T Cell Receptors (TCR)

• TCRs are specific for certain epitopes.

• Composed of alpha and beta chains on the plasma membrane, responsible for recognizing antigens.

  • Epitopes bind to regions on the alpha and beta chains.

• Associated proteins known as Cluster Differentiation (CD) markers aid T cell function.

CD Markers

• Cytotoxic T Cells (CD8+):

  • Interacts with MHC I through CD8 molecules.

• Helper T Cells (CD4+):

  • Interacts with MHC II through CD4 molecules.

VDJ Recombination

• TCR diversity is achieved via VDJ recombination during T cell maturation:

  • Scrambling and rejoining of gene segments to create unique receptors.

Activation of T Cells

• Cytotoxic T Cells Activation:

  • Requires recognition of antigen via TCR and presentation on MHC I.

  • Requires co-stimulation (binding of CD28 on T cell to CD80 on APC).

• Helper T Cells Activation:

  • Requires recognition of antigen via TCR and presentation on MHC II.

  • Also requires co-stimulation (binding of CD28 on T cell to CD80 on APC).

Immune System Regulation

• Co-stimulation is essential to prevent excessive immune responses that can lead to tissue damage.

• Over-activation can cause autoimmune-like reactions (e.g., organ failure).

Summary of T Cell Functions

• Cytotoxic T Cells:

  • Destroy infected cells by releasing perforins and initiating cell death.

  • Clone themselves for future immune responses.

• Helper T Cells:

  • Release cytokines to enhance function of immune cells.

  • Clone themselves for ongoing immune support.

Link Between T and B Lymphocytes

• Helper T cells (CD4+) play a key role in linking T cells and B cells:

  • B cells can present antigens via MHC II to helper T cells.

  • Activation of B cells leads to antibody-mediated immunity (humoral immunity).