Communication III (Immunological Control) - Bioengineering: A Conceptual Approach - page 84

for the pathogen bearing the epitope and the second is to confer immunity to the
human host. This is achieved by the effector cells and memory cells respectively.
Without B cells there are no antibodies, the two immune system components that
mediate the humoral response. Thus vaccination towards any pathogen must seek to
activate B cells.
The Cell Mediated Arm of Immunity
It can be argued that the cell mediated arm of immunity or cell mediated response
picks up where the humoral arm leaves off. Intracellular antigens, such as a virus
within an infected cell are not exposed to circulating antibodies and are therefore
inaccessible to the humoral response [ 8 ]. T cells, the major players in the cell medi-
ated response, probably evolved in response to this aspect of pathogenicity—the
need to combat intracellular parasites [ 13 ].
Like B lymphocytes, T lymphocytes are produced in the bone marrow; however
they migrate to the thymus, and organ in the upper chest, to undergo maturation
[ 13 , 15 ]. Also like B cells T cells undergo clonal expansion to form effector and
memory cells [ 15 ]. The primary difference between the two lymphocytes is that
unlike B cells, T cells never interact with native antigen or epitopes. On the contrary
T cells are stimulated for activity via a process known as antigen presentation.
Antigen Presenting Cells
T cells do not interact directly with native antigens. Rather processed antigens are
“presented” to T cells on their surfaces by a special groups of cells commonly
referred to as antigen presenting cells (APCs) [ 13 ]. APCs take up antigen, process
them via chemical reactions, and then place an antigenic fragment/epitope into a
specialized receptor on their surface [ 13 ]. The processed, surface bound epitope is
now fi t for interaction with T cells. APCs then migrate to specialized regions of the
immune system where T cells are abundant [ 13 ]. One primary location is the lymph
node through which lymphatic fl uid fl ows [ 13 ].
There are three major types of APCs all of which are WBCs: dendritic cells,
macrophage, and B cells [ 13 ]. Macrophages and dendritic cells are found predomi-
nantly in lymphatic tissue and fl uids and very important in their role as APCs [ 13 ].
Both ingest pathogens via phagocytosis—cell eating—and present pathogen specifi c
antigens on their surfaces [ 13 ]. B cells are a component of the humoral response and
their presentation of antigens is slightly different. B cells are not phagocytes; rather
they take in antigens via endocytosis—cell invagination—after the antigen binds
their antibody receptor [ 13 ]. All three types of APCs enhance the immune response
by their interaction with T cells following antigen ingestion.