The cytokines: The interferon family - Pharmaceutical Biotechnology: Concepts and Aapplications - page 209

Leukocytes

Mononuclear
phagocytes
monocyte/macrophages

Lymphocytes Granulocytes

Neutrophils
(PMN leukocytes)

Basophils

Null cells
(NK cells)

B-lymphocytes

T-lymphocytes

Eosinophils

T-cytotoxic
cells

T-helper cells T-suppressor cells

Figure 8.B1 The range of white blood cell types

their surface of high-affi nity receptors. In addition to blood, lymphocytes are present in high
numbers in the spleen and thymus. They may be subcategorized into antibody-producing B-
lymphocytes, T-lymphocytes (which are involved in cell-mediated immunity) and null cells.
T-lymphocytes may be subcategorized on a functional basis into T-helper, T-cytoxic and
T-suppressor cells. T-helper cells can produce various cytokines which can stimulate and
regulate the immune response. T-cytotoxic cells can induce the lysis of cells exhibiting for-
eign antigen on their surface. As such, their major target cells are body cells infected by
viruses or other intracellular pathogens (e.g. some protozoa). T-suppressor cells function to
dampen or suppress an activated immune response, thus functioning as an important 'off'
switch.
Most T-helper cells express a membrane protein termed CD4 on their surface. Most T-cyto-
toxic and T-suppressor cells produce a different cell surface protein, termed CD8. Monoclonal
antibodies specifi cally recognizing CD4 or CD8 proteins can thus be used to differentiate
between some T cell types.
Null cells are also known as 'large granular lymphocytes', but are best known as 'natural
killer' (NK) cells. These represent a third lymphocyte subgroup. They are capable of directly
lysing cancer cells and virally infected cells.
The third leukocyte cell type is termed granulocytes, due to the presence of large granules
in their cytoplasm. Granulocytes, many of which can be activated by cytokines, play a direct
role in immunity, and also in infl ammation. Granulocytes can be subdivided into three cell
types of which neutrophils (also known as polymorphonuclear leukocytes; PMN leukocytes)
are the most abundant. Attracted to the site of infection, they mediate acute infl ammation and
phagocytose opsonized antigen effi ciently due to the presence of an IgG F c receptor on their
surface. Eosinophils display a cell surface IgE receptor and, thus, seem to specialize in destroy-
ing foreign substances that specifi cally elicit an IgE response (e.g. parasitic worms). These
cells also play a direct role in allergic reactions. Basophils also express IgE receptors. Binding
of antigen-IgE complex prompts these cells to secrete their granule contents, which mediate
hypersensitivity reactions.