792
UNIT 4
Maintenance of the Body
21
the regulatory T cells, to suppress only those immune reactions
that lead to transplant rejection.
Check Your Understanding
19.
Which type of T cell is the most important in both cellular
and humoral immunity? Why?
20.
Describe the killing mechanism of cytotoxic T cells that
involves perforins.
21.
Which proteins must be carefully matched before an organ
transplant?
For answers, see Appendix H.
Homeostatic Imbalances
of Immunity
Give examples of immunodeficiency diseases and of
hypersensitivity states.
Cite factors involved in autoimmune disease.
(2) antiproliferative drugs, and (3) immunosuppressant drugs.
Many of these drugs kill rapidly dividing cells (such as activated
lymphocytes), and all of them have severe side effects.
±e major problem with immunosuppressive therapy is that
the patient’s suppressed immune system cannot protect the
body against other foreign agents. As a result, overwhelming
bacterial and viral infection remains the most frequent cause of
death in transplant patients. ±e key to successful graF survival
is to provide enough immunosuppression to prevent rejection
but not enough to be toxic, and to use antibiotics to keep in-
fections under control. Even with the best conditions, by ten
years aFer receiving a transplant, roughly 50% of patients have
rejected the donor organ.
Many research projects are seeking ways to induce tolerance.
One approach is to create a
chimeric immune system
(
chimer
5
monster) by temporarily suppressing the recipient’s bone
marrow and then dousing it with bone marrow from the same
donor as the new organ in the hope that this combined immune
system will treat the transplanted organ as self. Another ap-
proach tries to harness the body’s own tolerance-inducing cells,
Table 21.7
Cells and Molecules of the Adaptive Immune Response
ELEMENT
FUNCTION IN IMMUNE RESPONSE
Cells
B cell
Lymphocyte that matures in bone marrow. Induced to replicate by antigen binding, usually followed by helper T cell
interactions in lymphoid tissues. Its progeny (clone members) form plasma cells and memory cells.
Plasma cell
Antibody-producing “machine”; produces huge numbers of antibodies (immunoglobulins) with the same antigen
specificity. An effector B cell.
Helper T cell (T
H
)
An effector CD4 T cell central to both humoral and cellular immunity. It stimulates production of cytotoxic T cells and
plasma cells to fight invaders, activates macrophages, and acts both directly and indirectly by releasing cytokines.
Three major subsets (T
H
1, T
H
2, and T
H
17).
Cytotoxic T cell (T
C
)
An effector CD8 T cell. Activation usually requires helper T cell involvement. Its specialty is killing virus-invaded body
cells and cancer cells; also involved in rejection of foreign tissue grafts.
Regulatory T cell (T
Reg
)
Slows or stops activity of immune system. Important in controlling autoimmune diseases; several different populations
exist.
Memory cell
Descendant of activated B cell or any class of T cell; generated during initial immune response (primary response).
May exist in body for years after, enabling it to respond quickly and efficiently to subsequent infections or encounters
with same antigen.
Antigen-presenting
cell (APC)
Any of several cell types (dendritic cell, macrophage, B cell) that engulfs and digests antigens that it encounters,
then presents parts of them on its plasma membrane (bound to an MHC protein) for recognition by T cells bearing
receptors for the same antigen. This function, antigen presentation, is essential for activation of T cells and normal
cell-mediated responses. Macrophages and dendritic cells also release chemicals (cytokines) that activate many other
immune cells.
Molecules
Antigen
Substance capable of provoking an immune response. Typically a large, complex molecule (e.g., protein or modified
protein) not normally present in the body.
Antibody
(immunoglobulin
or Ig)
Protein produced by B cell or by plasma cell. Antibodies produced by plasma cells are released into body fluids
(blood, lymph, saliva, mucus, etc.), where they attach to antigens. This causes complement fixation, neutralization,
precipitation, or agglutination, which “marks” the antigens for destruction by phagocytes or complement.
Perforins, granzymes
Released by T
C
cells. Perforins create large pores in the target cell’s membrane, allowing entry of apoptosis-inducing
granzymes.
Complement
Group of bloodborne proteins activated after binding to antibody-covered antigens or certain molecules on the
surface of microorganisms; enhances inflammatory response and lyses some microorganisms.
Cytokines
Small proteins that act as chemical messengers between various parts of the immune system. See Table 21.6.
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