Page 323 - Anatomy and Physiology of Farm Animals, 8th Edition

308 / Anatomy and Physiology of Farm Animals

is also required for full activation of those embryonic development. In birds, lym-
phoid cells destined to become B cells leave
cytotoxic T cells. The activated cytotoxic T
VetBooks.ir cells also undergo a series of cell divisions to the bone marrow and travel to the bursa of
Fabricius, which is similar to the thymus
provide increased numbers of the selected
clone and also produce a subset to serve as but associated with the intestinal tract.
memory cells (memory T cells). Activated Here the cells undergo maturation and
C
cytotoxic cells (killer cells) destroy cells bear- selection similar to that which happens to
ing the antigen for which they were selected. pre‐T cells in the thymus. Mature B cells
Physical contact and binding between the leaving the bursa in birds also populate
cells bearing the antigen and the cytotoxic T peripheral lymphoid structures, and it is
cells are required for the destruction of anti- these structures that produce B cells in
gen‐bearing cells (Fig. 16‐1), and secretions adult birds. An organ that is the functional
from the cytotoxic T cells, including per- equivalent to the bursa has not been iden-
forin to disrupt the cell membrane of the tified in mammals. It is thought that B cells
antigen‐bearing cells, accomplish it. mature in the bone marrow in mammals
and leave the marrow to populate the
peripheral lymphoid structures. As with T
Lymphocyte Origin, cells, the peripheral lymphoid structures
Development, and Residence produce B cells in adult animals. The
development of NK cells probably occurs
Just as all lymphocytes are originally in a manner similar to that for B cells.
derived from pluripotent stem cells that
subsequently became lymphoid stem cells
in the bone marrow (see Fig. 15‐1), the Active and Passive Immunities
three general classes of lymphocytes (B
cells, T cells, and NK cells) are all derived Active immunity is a state of immunity to a
from lymphoid stem cells. specific antigen achieved by the response of
The differentiation of lymphoid stem one’s own immune system. Active immu-
cells to the types of mature lymphocytes nity may include both humoral and cell‐
begins early during embryonic develop- mediated responses. Passive immunity is a
ment. Cells destined to become T cells state of temporary immunity achieved by
leave the bone marrow and travel to the the transfer of immunoglobulins or T cells
thymus, where secretions called thymic from an animal with active immunity to
hormones act on them. These are not another that has not encountered the anti-
classic hormones, for they are local to the gen involved. Passive immunity is a tempo-
thymus, and lymphocytes must come to rary state because the immunoglobulins
the thymus to be acted on. The thymic and/or T cells are degraded or destroyed
hormones guide the further development over time. Passive immunity is important in
of the lymphocytes to become T cells. As neonates that utilize immunity from the
part of the development in the thymus, dam while their own immune system is too
only T cells capable of recognizing foreign immature to provide active immunity.
antigens are selected to survive. The
selected T cells that leave the thymus pop-
ulate lymphoid structures throughout the Immunological Surveillance
body, including tonsils, lymph nodes, the
spleen, and collections of lymphocytes in NK cells and cytotoxic T lymphocytes (killer
the intestinal wall. These structures are cells) both recognize and destroy cells with
responsible for producing T cells in adult foreign antigens in their membranes. Cancer
animals (discussed later). cells have surface antigens that are not
Lymphoid stem cells destined to become present in normal cells and thus can be rec-
B cells also undergo differentiation during ognized as foreign. It is believed that cancer