Immunology
Human body has several mechanisms
that defend against invasion and colonization of foreign organism. This defensive
ability is called immunity. Immunology is the study of the ways in which the
body defends itself from infectious agents and other foreign substances in its surrounding
environment.
Immune Cells
Immune cells arise from
hematopoietic stem cells (HSC), which resides in the bone marrow and have unique
ability to give rise to all of the different mature blood cell types under
favorable conditions.
The progeny of HSCs initially
commit to one of three main alternatives differentiation pathways (lineage)
that yield erythrocytes, lymphocytes or myeloid cells, respectively. The mature
cells of myeloid lieage includes neutrophils, monocytes, mast cells,
eosinophils, basophils, and megakaryocytes.
Mature cells of the lymphatic lineage
include B-lymphocytes, T-lymphocytes, and also Natueal killer cells (NK).
Phagocytes
All types of leukocytes
contribute to host defense, three types play a special prominent roles, two of these
(the neutrophils and the monocyte-macrophage series) are phagocyte cells, which
act primary by engulfing and digesting bacteria, cellular debris etc. The third
groups, made up of lymphocytes and their relative, have a little phagocytic capacity
but carry out a host of other protective reactions, known as immune responses. Phagocytes
are able to recognize and kill pathogen directly and are therefore considered
as the most important cellular effectors of the inmate immune system.
Neutrophils
Neutrophils grow and mature in
bone marrow, they are then released into blood stream. An adult has
approximately 50 billion neutrophils and each cell dies by approximately 12 hours
after entering into the blood stream. However bone marrow continues to produce
neutrophils to maintain the stable circulation of neutrophils in the blood.
If the neutophile finds an
inflamed tissue while circulating in the blood, the cells rapidly adhere to the
activated endothelium of the local post capillary venules, migrate through the
wall of the vessels and invade the affected tissues and neutrophil immediately
begin the process of engulfing any bacteria, cellular debris or foreign bodies
in the area.
Mononuclear Phagocytes: The Monocyte-Macrophage System
A tissue macrophage lives
approximately 2-4 months. During this time, some macrophages remain immobile
and others wander incessantly by ameboid motion. In either case the cells continuously
monitor the surrounding by pinocytosis through an extensive array of receptors
on its surface. Whenever it encounters certain inflammatory mediators or other
signals of tissue distress, the cells undergo a process known as macrophage
activation, in which bacteria and other pathogens are killed more efficiently.
Lymphocytes
The typical lymphocyte is a
small, round or cuboidal in shape, spherical nucleus,. The most fundamental
distinction is the division of these cells into two major lineages known as
T-cells and B-cells.
B- Lymphocytes
The definitive feature of the
cells in the B-cell lineage is their ability to synthesize protein called immunoglobulin.
No other cells express these proteins.
Mature B cells can express
immunoglobulin in two different forms that each serve as unique function. In
resting (naïve or memory) B lymphocytes, immunoglobulins are expressed only on
the cell surface, where they serve as membrane-bound receptors for specific
antigens. In contrast, the effector cells of the B lineage (plasma cells) are
uniquely specialized to secrete large amount of immunoglobulin protein into
their surrounding milieu. The secreted immunoglobulins retain the ability to
recognize and bind their specific ligands and are often referred as antibodies.