LESSON 3
CELLS OF THE IMMUNE SYSTEM
Section I. INTRODUCTION
3-1.
LYMPHOID TISSUES
Acquired immunity is the work of the body's lymphoid tissues. Lymphoid tissue
can be divided into two major groups. 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. Lymphoid development is antigen-dependent in
these areas.
3-2.
LYMPHOCYTES
When studied with a microscope, most of the lymphocytes found in the lymphoid
tissue look pretty much alike. However, there are two distinct types when studied
further. One group is responsible for cellular immunity; they are called T cells because
they must be preprocessed in the thymus gland. The other group, whose purpose is to
form antibodies, is called B cells, because they were first discovered in birds (in which
B cells are preprocessed in a structure unique to birds, the bursa of Fabricius).
3-3.
DEVELOPMENT OF B CELLS AND T CELLS
a. Two types of lymphocytes are produced, and the bone marrow (and fetal
liver) is the site of origin. These lymphocytes are called T lymphocytes and B
lymphocytes. 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.
b. Precursor T cells 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.
c. Changes in T cell specific surface markers occur at various stages of T cell
development. The end stage of differentiation results in two distinct T cell subsets:
(1) those which express T4 (helper/inducer T cells) and (2) those which express T8
(suppressor/cytotoxic T cells). Helper versus suppressor T cells will be discussed in
more detail later in this lesson. Monoclonal antibodies have been produced to several T
cell specific antigens.
MD0838
3-2