78 Steps Health Journal

Destruction of T Lymphocytes

The activated T lymphocytes must be destroyed after the infection has been cleared. This occurs because T cells produce a surface receptor called FAS. Production of FAS increases during the infection and, after a few days, the activated T lymphocytes begin to produce another surface molecule called FAS ligand. The binding of FAS to FAS ligand, on the same or on different cells, triggers the apoptosis (cell suicide) of the lymphocytes.

^R- Glucocorticoids (such as hydrocortisone) secreted by SP I t the adrenal cortex can act to inhibit the activity of the 1 ^ immune system and suppress inflammation. This is why cortisone and its analogues (such as Prednisone) are used clinically to treat inflammatory disorders and to inhibit the immune rejection of transplanted organs. The immunosuppressive effect of these hormones may be due to the fact that they inhibit secretion of the cytokines. It is interesting in this regard that inter-leukin-1 (IL-1), which can be produced by microglia in the brain, has been shown to stimulate the pituitary-adrenal axis by promoting CRH, ACTH, and glucocorticoid secretion (chapter 11). In a negative feedback fashion, the glucocorticoids then inhibit the immune system and suppress the production of the inflammatory cytokines, including IL-1, IL-2, and TNF a. These and related observations have opened up a new scientific field devoted to the study of interactions between the nervous, endocrine, and immune systems.

Clinical Investigation Clue

Remember that Gary was given cortisone for his rash. Why would cortisone be useful for treating Gary's rash?

Test Yourself Before You Continue

1. Describe the role of the thymus in cell-mediated immunity.

2. Define the term cytokines, state the origin of these molecules, and describe their different functions.

3. Define the term histocompatibility antigens and explain the importance of class-1 and class-2 MHC molecules in the function of T cells.

4. Describe the requirements for activation of helper T cells by macrophages. Explain how helper T cells promote the immunological defenses provided by killer T cells and by B cells.

Fox: Human Physiology, Eighth Edition

15. The Immune System

Text

© The McGraw-H Companies, 2003

The Immune System

Viral antigen Class-2 MHC molecule

■ Figure 15.19 The interaction of macrophages, helper T cells, and killer T cells. This sequence leads to the activation of killer T cells, and thus the destruction of infected cells in the defense against viral infections.

Viral antigen Class-2 MHC molecule

■ Figure 15.19 The interaction of macrophages, helper T cells, and killer T cells. This sequence leads to the activation of killer T cells, and thus the destruction of infected cells in the defense against viral infections.

Infected cell destroyed

Infected cell

Killer T cell

Infected cell destroyed

Infected cell

Killer T cell

Activated helper T cell

Foreign antigen Class-2 MHC molecule

Helper T cell

Foreign antigen Class-2 MHC molecule

Helper T cell

Foreign antigen

Class-2 MHC molecule y

Activated helper T cell

B cell

■ Figure 15.20 The interaction of macrophages, helper T cells, and B cells. A schematic representation of the interactions that can be involved in the activation of B cells.

Foreign antigen

Class-2 MHC molecule y

B cell

> >

Specific antibody

Plasma cell