Cells of the Immune System
Morphology and staining characteristics of various types of blood cells. Red
blood cells and platelets, which both lack nuclei, are the most numerous. Most numerous of
the leukocyte populations are the neutrophils. Lymphocytes are the predominant cell type
responsible for immune responses. Move the mouse pointer over the cell types illustrated and click the mouse
button for distinguishing features.
The immune system is constituted by a surprising
variety of different cell types disseminated throughout the body and collectively defining
one's capacity to mount an immune response. All blood cells and certain other cells
located throughout the body - particularly in the reticuloendothelial
system (RES) - are continuously regenerated throughout
life by the process called hematopoiesis. Most hematopoietic
cells are short lived, some surviving for only a day or two, and thus hematopoiesis serves
to maintain a steady renewal of these cells on physiological demand. Hematopoiesis is
believed to be the function of a single precursor cell called the pluripotent stem cell (SC). In
addition to being self-renewing, SCs undergo multi-lineage differentiation
driven by stage-specific cytokine and cell-cell interactions.
In general, hematopoietic cells are highly
mobile, moving with the flow of blood in the cardiovascular system or with the flow of
lymph in the lymphatic system. Many of these cells also cross-migrate between these two
circulatory systems, and many migrate directly into the tissues, particularly under the
influence of inflammatory cytokines with chemoattractive properties. High concentrations
of hematopoietic cells, particularly lymphocytes, are also localized in the primary lymphoid organs, i.e., thymus
and bone marrow, and secondary
lymphoid organs, e.g., spleen, lymph nodes, etc.
Obviously, the task of identifying different
hematopoietic cells is a special challenge considering the sheer diversity of these cells.
Most of the basic cell types can be distinguished on a gross morphological level using
various cytochemical staining techniques. However, some of the
basic cell types are comprised of heterogeneous mixtures of functionally distinct
subpopulations that are morphologically indistinguishable. In this case, immunologists
resort to monoclonal antibodies directed against specific cell-surface molecules called
CD antigens. CD antigens are differentially expressed on leukocytes
and distinct cell subpopulations can be identified and even isolated according to their
patterns of CD antigen expression.