Antigen Presenting Cells and Dendritic Cells

Antigen Presenting Cells (APCs)

• APCs are immune cells that display antigens on their surface, alerting other immune cells to an infection.

• APCs can be divided into two groups:

  • Professional APCs: These are cells specialized for presenting antigens, including dendritic cells, macrophages, and B cells. They are highly efficient at antigen presentation and activating naive T cells.
  • Non-professional APCs: These cells can also present antigens but only under specific circumstances, such as during an infection. They are less effective at antigen presentation compared to professional APCs.

• Professional APCs are important because they:

  • Display antigens on their surface for T cells to recognize.
  • Provide signals necessary to fully activate naive T cells.

Types of Dendritic Cells

• Langerhans cells: Found in the skin and mucosal epithelium. Capture antigens from these surfaces and migrate to lymph nodes to trigger an immune response.

• Interdigitating cells: Dendritic cells that travel to lymphoid tissues (like lymph nodes) and interact with T cells. They help activate T cells by presenting antigens.

• Follicular dendritic cells (FDCs): Found in the primary and secondary follicles of B cell areas in lymph nodes and the spleen. Present antigens to B cells to aid in antibody production.

• Germinal center dendritic cells (GCDC): Found within germinal centers. Assist in B cell maturation and the selection of high-affinity antibodies, forming memory B cells.

Dendritic Cell Migration and Maturation

• When dendritic cells encounter an antigen, they activate and undergo various changes:

  • Loss of adhesiveness to the epithelial tissue.
  • Expression of specific surface receptors that respond to chemokines.
  • Migration to lymph nodes guided by chemokines.

• As dendritic cells mature, they express MHC and co-stimulatory molecules for efficient interaction and activation of T cells.

MHC Expression in Professional APCs

• Macrophages: MHC-II expression is induced by bacteria and/or cytokines. MHC-I is expressed at low levels.

• Dendritic Cells: MHC-II is expressed constantly and at high levels. MHC-I is expressed at low levels but is induced by bacteria and/or cytokines.

• B Cells: MHC-II is always expressed and is further induced upon activation. MHC-I is constantly expressed at low levels.

Antigen Presentation by Professional APCs

• Macrophages: Process both intracellular and extracellular antigens. Antigens are presented via MHC-II (extracellular antigens) and MHC-I (intracellular).

• Dendritic Cells: Process extracellular antigens. Antigens are presented via MHC-II.

• B Cells: Process extracellular antigens that bind to specific immunoglobulin receptors. Antigens are presented via MHC-II.

Co-stimulation

• Co-stimulatory molecules (like B7 expression) are necessary for full T cell activation. B7 expression is induced by bacteria and/or cytokines in macrophages. In dendritic cells, B7 is always expressed at high levels. B7 expression in B cells is induced upon activation.

Location of Professional APCs

• Macrophages: Found in connective tissues, lymphoid tissues, body cavities, and blood.

• Dendritic Cells: Located in the connective tissue, epithelium, and lymphoid tissues.

• B Cells: Found in lymphoid tissues and blood.

Autoimmunity

• Autoimmunity occurs when the immune system mistakenly attacks the body's own tissues and organs.

• Mechanisms of autoimmunity:

  • Tissue damage: Damaged tissue releases antigens that are mistaken for foreign invaders.
  • Molecular changes: Self-proteins may undergo structural changes and be recognized as foreign.
  • Molecular mimicry: Foreign antigens resemble self-proteins, leading to cross-reactivity.
  • Alteration in antigen processing: Defects in antigen processing can lead to improper recognition of self-antigens.
  • Infection: Certain infections can disrupt tolerance to self-antigens.
  • Genetic factors: Specific genes can increase susceptibility to autoimmune diseases.

• Other contributing factors:

  • Lymphocyte abnormalities: Abnormal B or T cells can contribute to autoimmunity.
  • Failure of central tolerance: Self-reactive lymphocytes fail to be eliminated in the thymus or bone marrow.
  • Overcome of peripheral tolerance: Mechanisms that suppress self-reactive lymphocytes fail.
  • Polyclonal lymphocyte activation: Non-specific activation of lymphocytes, potentially including self-reactive ones, can lead to autoimmunity.

Explore the essential roles of antigen presenting cells (APCs) in the immune response, including the distinction between professional and non-professional APCs. Gain insights into the specific types of dendritic cells, such as Langerhans cells, and their functions in triggering immunity. Test your understanding of these key concepts in immunology.