Figure 1-14: Cellular Interactions


Cellular interactions involved in induction of immune responses.

  1. Activation and proliferation of TH cells (a) is required for
  2. generation of humoral response (b) and
  3. cell-mediated response to altered self-cells (c).
APC = antigen-presenting cell; Ag = antigen.

© 1997, Duane W. Sears


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The term "humoral" is derived from the Latin humor, meaning "body fluid"; thus humoral immunity refers to immunity that can be conferred on a nonimmune individual by administration of serum antibodies from an immune individual.

The humoral branch of the immune system involves interaction of B cells with antigen and their subsequent proliferation and differentiation into antibody-secreting plasma cells. Antibody functions as the effector of the humoral response by binding to antigen and neutralizing it or facilitating its elimination. When an antigen is coated with antibody, it can be eliminated in several ways.

  • For example, antibody can cross-link the antigen, forming clusters that are more readily ingested by phagocytic cells.
  • Binding of antibody to antigen on a microorganism also can activate the complement system, resulting in lysis of the foreign organism.
  • Antibody can also neutralize toxins or viral particles by coating them and preventing their subsequent binding to host cells.

In contrast, cell-mediated immunity can be transferred only by administration of T cells from an immune individual.

Effector T cells generated in response to antigen are responsible for cell-mediated immunity. Both activated TH cells and cytotoxic T lymphocytes (CTLs) serve as effector cells in cell-mediated immune reactions.

  • Cytokines secreted by TH cells can activate various phagocytic cells, enabling them to phagocytose and kill microorganisms more effectively. This type of cell-mediated immune response is especially important in host defense against intracellular bacteria and protozoa.
  • CTLs participate in cell-mediated immune reactions by killing altered self-cells, they play an important role in the killing of virus-infected cells and tumor cells.

W. H. Freeman & Co. and Sumanas, Inc.
Immunology, January, 1997

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version: 4 December 2015