280
Lymphoid System and
Cellular Immunology
Answers
167.The answer is a.(Alberts, pp 1368–1369. Kindt, pp 7, 16–18. Abbas,
pp 7, 72, 269.)Clonal selection is the immune system’s way of fine-tuning
the immune response. Clonal selection is the means by which the immune
system makes antigen receptor sites (T cells) or antibodies (B cells) more
and more specific to create the optimal response to one specific antigen.
In clonal selection a clone of lymphocytes is committed to respond to
a particular antigen. The antigenic determinants, which consist of specific
amino acids or monosaccharides, actually induce many clones and a wide
variety of humoral and cell-mediated responses. This occurs during the
development and maturation of the immune system and is responsible for
the specificity of lymphocyte cell surface receptors for antigens.
Immunoglobulin class switching (answer b)occurs during the maturation
of B cells after antigen stimulation. Homing receptor differentiation
(answer e)is an important part of naïve T- and B-cell maturation in the
thymus and bone marrow, respectively. Regulatory T cells (Treg) actively
suppress activation of the immune system and inhibit pathological self-
reactivity and autoimmune disease (answer c). Clonal selection does not
lead to formation of CFUs (answer d).
168.The answer is a.(Abbas, pp 184–190. Kindt, pp 426–434.)Transplant
(acute graft) rejection is mediated by the action of T cells. Helper T (TH)
cells recognize peptides associated with MHC antigens from the donor tissue
and become activated. Activated THcells release interleukin 2 (IL-2) and
interferon-γ(IFN-γ), which activate cytotoxic T (TC) cells, B cells, and
macrophages. Although all of these cells may be involved in the rejection
process, TCcells are the primary agents of transplant rejection in an acute
graft rejection response. This has been confirmed by animal studies: ani-
mals deficient in T cells are incapable of rejecting grafts. Tacrolimus
(FK506) blocks signal transduction pathways leading to IL-2 production
and therefore preventing T cell activation.