Structure and Functions of the Immune System
The lymphoid system consists of the lymphoid cells (lymphocytes
and plasma cells) and lymphoid organs. Based on different roles they
perform, lymphoid organs can be classified into central (primary) and
peripheral (secondary) lymphoid organs. The central lymphoid organs are
lymphoepithelial structures in which the precursor lymphocytes proliferate,
develop and acquire immunological capability. In mammals, the bone
marrow, the thymus and the bursa of fabricius in birds represent primary
lymphoid organs. (Structure and Functions of the Immune System)
After acquiring immunocompetence, the lymphocytes migrate along blood and lymph streams, accumulate in the peripheral lymphoid organs and, following antigenic stimulus, effect the appropriate immune response. The spleen, lymph nodes and mucosa – associated lymphoid tissue (MALT) constitute the major peripheral or secondary lymphoid organs. Lymphoidal tissues in the gut ( peyer’s patches), appendix, tonsils, salivary glands, tear glands and also the secretion
(colostrums) of the lactating breast of the mother also are included in the
immune sysytem. (Structure and Functions of the Immune System)
Primary Lymphoid Organ
Thymus :
The thymus develops at about the sixth week of gestation. By eighth
week, it grows into a compact epithelial structure. Mesenchymal stem cells
(precursors of lymphocytes) from the yolk sac, foetal liver and bone marrow
reach the thymus and differentiate into the thymic lymphoid cells (thymocytes).
The thymus acquires its characteristic lymphoid appearance by the
third month of gestation. It is thus the first organ in all animal species to become predominantly lymphoid. In human beings, the thymus reaches its
maximal size just prior to birth. Thymus continues to grow till about the 12th
year. After puberty, it undergoes spontaneous progressive involution,
indicating that it functions best only in early life. (Structure and Functions of the Immune System)
The thymus is located just behind the upper part of the heart. It has
two lobes surrounded by a fibrous capsule. Septa arising from the capsule
divide the gland into lobules which are differentiated into an outer cortex
and an inner medulla. The cortex is crowded with actively proliferating
small lymphocytes. The medulla consists mainly of epithelial cells and mature
lymphocytes amidst which are the hassall’s corpuscles, which are whorl-like
aggregations of epithelial cells. (Structure and Functions of the Immune System)
The thymus was considered as an organ without any recognized
function. But its role in the development of cell mediated immunity has been
found recently. The primary function of the thymus is the production of
thymic lymphocytes(T cells). It is the major site for T lymphocyte
proliferation in the body. (Structure and Functions of the Immune System)
However, of the lymphocytes produced, only about one per cent leave the thymus. The rest are destroyed locally by programmed cell death or apoptosis. In the thymus, the lymphocytes acquire new surface antigens (Thy antigens). Lymphocytes produced in the thymus are called ‘thymus (T) dependent lymphocytes’ or ‘T cells’. Unlike, lymphocytes proliferation in the peripheral organs, the function of thymus is independent of antigenic stimulation. (Structure and Functions of the Immune System)
The thymus confers immunological competence on the lymphocytes
during their stay in the organ. Prethymic lymphocytes are not
immunocompetent. In the thymus they are ‘educated’ so that they become
capable of mounting cell mediated immune response against appropriate
antigens. This is effected by hormone-like factors produced by the thymus
such as thymulin, thymosin and thymopoietin. (Structure and Functions of the Immune System)
The importance of thymus in lymphocyte proliferation and development of CMI is evident from the effects of lymphopenia, and in neonatally thymectomised mice.
T lymphocytes are selectively seeded into certain sites in the peripheral lymphatic tissues. These are found in the white pulp of the spleen, around the periarteriole region, and in the paracortical areas of lymph nodes. (Structure and Functions of the Immune System)
Related Topics in Zoology:
Bio Zoology All Important Topics
- Microbiology Introduction and History of Medical Microbiology
- Pasteur, Koch, Lister
- Structure of Viruses
- Viral genetics
- Virus Culture
- Viral Diseases
- Bacteria Structure Culture
- Bacterial Genetics
- Bacterial Diseases
- Protozoan microbiology
- Pathogenecity of Microorganisms
- Antimicrobial Resistance
- Antibiotics and Chemotherapy
- AIDS – HIV