SOLUTIONS TO EXERCISES, LESSON 3
Acquired immunity is the work of the body's lymphoid tissues. Lymphoid tissue
can be divided into two major groups: the central lymphoid organs and the
peripheral lymphatic tissue. The central lymphoid organs consist of the bone
marrow and thymus (and the fetal liver). In these areas, stem cells give rise to
proliferating and differentiating lymphocytes through processes completely
independent of antigen stimulation. The peripheral lymphatic tissue includes
lymph nodes, spleen, and gut-associated lymphoid tissue.
One group of lymphocytes called T cells are responsible for cellular immunity.
They are called this because they must be preprocessed in the thymus gland. The
other group, whose purpose is to form antibodies, is called B cells.
While the major role of the bone marrow in adults is to replenish blood cells, it also
serves as a protected environment in which T and B lymphocytes undergo
antigen-independent proliferation. Precursor T cells then move through the
bloodstream and pass through the walls of blood vessels to the thymus. They
rapidly proliferate within the gland and acquire new surface markers. T cells pass
from the thymus to the blood and seed peripheral lymphoid tissue, where they
begin to function as immunocompetent T cells.
At the end stage of T cell differentiation, there are two distinct subsets of T cells:
(1) helper (inducer) T cells, which express T4 and (2) suppressor (cytotoxic) T
cells, which express T8.
Maturation of B cells in humans takes place first in the fetal liver and later in the
bone marrow of the adult.
Monoclonal antibodies can be used to detect B-specific markers. At various
stages of maturation, a B cell expresses unique markers on its surface that are
characteristic of a particular developmental stage.