There is nearly always a circulating lymphocyte that can bind to an antigen that has
become associated with a major histocompatibility complex (MHC) class I or class II
molecule.
There are different T cell phenotypes which are distinguished from one another by their
surface marker molecules. Firstly there are T helper (Th) cells that have a surface mole-
cule known as the CD4 (CD4+) in close association with the CD3 molecule. Secondly
there are T cytotoxic (Tc) cells that have CD8 molecules (CD8+) in association with the
CD3 receptor and thirdly T suppressor (Ts) cells have both CD4 and CD8 markers.
In addition there is another separate group of lymphocyte type cells: the natural killer
(NK) cells, which are granular lymphocyte-like cells.n 5.6. 3MACROPHAGES AND PHAGOCYTOSIS
The first cells to respond positively and even aggressively to non-self material are the phago-
cytic cells. The most common phagocytic cell is a macrophage and originates from a pre-
cursor in the bone marrow. From the bone marrow the cell enters the blood circulation
as a monocyte and when recruited into the tissues it differentiates into a macrophage.
Phagocytosis is the process whereby the macrophages engulf non-self material includ-
ing dying and discarded host cells. Macrophages specialise in phagocytosis and have been
observed in vitro to engulf microscopic particles as well as microbes. The engulfed microbes
are attacked in macrophage cytoplasm by reactive oxygen and nitrogen metabolites.
When macrophages make contact with foreign material they secrete cytokines and
chemokines, which are the signals that set an immune response in motion. The first
response is that of an innate immune response and if the infection is not resolved it
becomes chronic and the response merges into an adaptive immune response. Once the
presence of a pathogen is detected both the macrophages become activated and other cells
such as mast cells are also stimulated and consequently release chemical mediators of
inflammation. Hence the macrophages not only secrete inflammatory mediators but also
respond to those derived from other cell sources.
The wound healing that follows inflammation is inaugurated by proteases and growth
factors secreted by macrophages. Macrophages engulf, digest and process the pathogen and
break it down into peptide fragments. Some of the peptides are transported to the macro-
phage cell membrane where they become combined with MHC class II molecules. These
macrophages migrate to the lymph nodes to become antigen-presenting cells. Those
microbes able to survive destruction within the macrophages, that is they can evade the first
stage of the immune response, can cause tissue damage and often are the cause of diseases.n 5.6.4 MAST CELLS
Mast cells are granular cells that originate from stem cells in the bone marrow. Mature
mast cells are nucleated granular cells that congregate in connective tissues throughout
the body. These cells play an important role in the both innate and adaptive immune
responses. On their external membranes are receptors for antibodies and eosinophils as
well as sensors for the detection of any alterations in the form of damage, temperature,
oxygen concentration or the presence of pathogens. Any of these changes can stimulate
the mast cells to degranulate. The granules contain the precursors to numerous phar-
macologically active compounds. Among the most prominent are: histamine, serotonin,
bradykinin, 5-hydroxytryptamine (5-HT), slow reacting substance (SRS) etc.
Mastocytosis, an increase in the number of mast cells, is a phenomenon associated with
parasitic infections and allergies. As with eosinophils there is a debate about some of the
exact functions of mast cells.PARASITOLOGY