To test the response of lymphocytes to a specific antigen that is being presented, the lymphocyte stimulation test (Fig. 1) may be used. This test measures the response of the T cells to antigen as indicated by their entering the cell cycle and incorporating precursors of DNA synthesis.
The lymphocyte stimulation test
Fig. 1 In the lymphocyte stimulation test, whole blood in saline solution is first layered on Ficoll Isopaque (which has a density between, and therefore separates, white cells and red cells) and centrifuged (400 g). This separates the
lymphocytes from the other cell and serum constituents. The cells are washed (to remove contaminants such as antigen) and then put into test tubes with a suspension of antigen and culture medium. Tritiated thymidine (3H-thymidine) is added
16 hours before the cells are harvested. The cells are harvested on a glass-fiber filter disk and their radioactivity measured by placing the disk in a liquid scintillation counter. A high count indicates that the lymphocytes have undergone transformation and confirms their responsiveness to the antigen. This test can also be used for cells from lymphoid tissue. 3H- thymidine culture medium whole blood and saline Ficoll Isopaque lymphocytes separated blood cultivation glass fiber filter measure radioactivity centrifugation antigen suspension
Cells at the end stage of differentiation, such as plasma cells, may become so specialized that they lose surface molecules such as MHC class II, and are unable to respond to regulatory signals or to proliferate.
The fate of lymphocytes responding to antigen is varied:
• some can persist for a long time as memory cells – the life span of memory cells can be more than 40 years in humans, as judged by the chromosome abnormalities (e.g. cross-linking of DNA which would prevent mitosis) found in the blood cells of Hiroshima survivors;
• other lymphocytes have a short life span, which explains why moderate antigenic stimulation does not lead to lymphoid enlargement – this is nevertheless sufficient for generating effective cell-mediated and antibody responses.
Apoptosis is critically important for disposing of unwanted cells after an immune response.
The immune system responds to clues that an infection has taken place before
responding strongly to antigens
In recent years it has become appreciated that APCs must respond appropriately to an infection, for example, but not to high levels of harmless substances that may fluctuate in the environment.
APC activation is generally a response to infection, or at least the presence of substances, such as constituents of bacterial cell walls, characteristic of infection. This requirement neatly explains the need for adjuvants, which are typically derived from bacterial components.
Adjuvants are generally necessary in vaccines to stim- ulate a robust immune response. The concept of immune activation only in response to infection (or adjuvant as a surrogate for infection), and not to other antigens, has been popularized as the ‘danger’ hypothesis. This idea proposes that the immune system does not merely distinguish self from non-self, but responds to clues that an infection has taken place before responding strongly to antigens.
In other words, foreign substances may be innocuous or invisible to the immune system unless accompanied by danger signals, such as infection. These danger signals are provided by receptors for microbial products on APCs, such as the Toll-like receptors (TLRs, see Fig. 6.24).
FURTHER READING
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FURTHER READING
Expression of the high-affinity IL-2 receptor on T cells
Fig. 7.23 The high-affinity IL-2R consists of three polypeptide chains, shown schematically. Resting T cells do not express the α chain, but after activation they may express up to 50 000 α chains per cell. Some of these associate with the β chain to form the high-affinity IL-2R.
approximate numbers per cell at rest maximum activation 5000 0 ~5000 500 5000 50 000 chain chain chain
55 kDa 75 kDa 64 kDa
– affinity 10 10 M –– affinity 10 11 M affinity very low in isolation affinity 10 8M IL-2
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