Immune system


What is Immune system? Types of Immune system? Organs of the immune system | Primary lymphoid organs | Secondary lymphoid organs |

Immune system


Cells of immune system:-

One of the major ways through which the immune system operates by producing different kinds of cells. All the cells of immune system develop from the same group of stem cells of bone marrow.

Stem cells: –

The stem cells are the precursor cells of lymphocytes. The hematopoietic stem cells are known as hemocytoblasts. These are undifferentiated cells, immunologically non component. They retained the embryonic ability to differentiate in to all types of blood cells. The stem cells are large cells with the rim of cytoplasm large nucleus. The stem cells are capable of both self replication, generation of series of progenitor cells by proliferation and differentiation under the influence of factors known as colony stimulating factors-

  • Granulocyte-macrophage stimulating factor.
  • Eosinophyl colony stimulating factor.

The colony stimulating factor acts as stem cells resulting in the formation of respective cells of progenitor cells.

Lymphocytes: –

Lymphocytes are the central cells of the immune system responsible for the acquired immunity. Lymphocytes constitute 22 – 40% of the body white blood cells and 99% of the cells in the lymph. These are approx. 1010 -1012lymphocytes in the human body. These lymphocytes continuously circulate in the blood and lymph and are capable of migrating into the tissue spaces the and lymphoid organs, thereby providing a high degree of immunity.

They are broadly classified on the basis of function and cell membrane components into three populations- B-cells, t cells and null cells


B-cells {or} B-lymphocytes:-

The B-lymphocytes derived its name from its site of maturation in the bone marrow. Matured B-cells are characterized by the presence of membrane bound immunoglobulin { antibody} molecule which acts as receptor for the antigen. Appropriate interaction between antigen and membrane bound antibody on b-cells, B-cells divide repeatedly and differentiatey into a population of plasma cells and memory cells. Plasma cells are the actual cells that secrete the antibody.

B-cells {or} B-lymphocytes
B-cells {or} B-lymphocytes

T cells {or} T-lymphocytes:-

T-lymphocytes derive their name from the site of maturation in the thymus like b-cells, these cells have membrane bound receptors for antigen called t cell receptor. T cell receptor recognise antigen only when antigen is associated with complex called mhc- major histocompatibility complexes.

T cells that expresses cdf receptor, recognise the antigen associated to class II mhc molecules, where is the t cells that express cds receptors recognise the antigen associated with class i mhc molecules. Cd4+ t cells generally functions as t helper {th} cell and cds + t cells generally function as t cytotoxic {tc} cells. Another sub-population of t cells called t suppressor { ts} cells has been postulated.

T cells are also activated by the interaction of antigen with class i mhc molecules, cytotoxic activity and estimated the antigen.


Null cells:-

A small group of peripheral blood lymphocytes called null cells. Were observed one functional population of null cells called natural killer cells are large, granulated lymphocytes constitue 5 to 10% of lymphocytes in humans. Nk cells was first described in 1976, display cytotoxic activity against a wide range of tumor cells. These null cells lack the membrane bound receptors.



The monocytes are grouped under mono nuclear cells. The mono nuclear phagocytic system consists of circulating monocytes in the blood. During haemotopoiesis in the bone marrow, monocytes progenitor cells differentiate into pro monocytes leave the bone marrow and enter the blood, differentiate into mature monocytex. Monocytes circulate in the bloodstream about 8 hours, during which type and enlarge, then they migrate into the tissue and differentiate into specific tissue macrophages.


Differentiation of monocytes into a tissue macro-phage involves number of changes. The cell in enlarges 5 to 10 folds in its cell organelles, size, production of variety of lytic enzyme etc.,

The specific tissue macrophages are named as loading to their location such as:-

  • Alveroar macrophages in lungs
  • List of sites in connective tissue
  • Kupper cells in liver
  • Mesoglial cell in kidney
  • The macrophages are most effective in eliminating real pathogen because they exhibit greater phagocytosis activity.


Organs of the immune system

Write a detailed account of the organs of the immune system ?

The cell involved in the immune reactions are organised into the tissue and organ which are collectively known as lymphoid system. This system performs the major functions of protecting against the various microorganisms that enter the body through all the possible routes. It is formed by lymphoid cells and lymphoid organ. Which are mainly composed of lymphocytes, occur either as discrete, capsulated organs as thymus, spleen etc., based on the structural and functional stages of lymphoid organs.

They can be classified into two groups namely.

Primary lymphoid organs.

Secondary lymphoid organs.

Primary lymphoid organs:–

Lymphoid organs are the major sites for lymphpoeisis here the lymphoid stem cells proliferate, differentiate and mature immunocompetent cells in the absence of antigenic stimulation. Thymus and bursa fabricus { in birds}, in mammals, B-cells maturation occur in the bone and t cells maturation occur in the thymus.


In mammals the thymus is originated from the epithelium of pharyngel pouches. By about sheath it loses its connection from the pharynx. Lateral epithelial structure is filled up by the mesenchymal lymphoid cells formed from the foetal liver and bone marrow. It is fully developed organ at birth. After birth the growth of the animal, it also increases size and reaches the maximum size. Then it involves into ie., it gradually decreases in size and become almost invisible in adult stages

In human beings the thymus occurs as two oval lobes visit behind top of the thyroid gland along the neck. At the time of birth it’s weight about 15 to 20 grams and increase in size after birth it reaches about 40 grams in weight by puberty and their after it slowly disappears.

The bilobed thymus is covered by a fibrous capsule. Each lobe of thymus is organized into lobules which are separated from one another by septa arising from the capsule called trabeculae within each lobule. The cells are arranged into an outer cortex and and inner medulla the cortex is tightly packed with proliferating immature lymphocytes while the medulla contains more mature cells.

The epithelial cells of the thymus are known to synthesize a group of thymus hormones. Such as thymulind, thymosin and thymopoietin. These hormones are found to bring about the transformation of lymphocytes to T-lymphocytes. The T-lymphocytes are not morphologically distinguishable from B-lymphocytes. But they differ from B-lymphocytes presence of surface marker antigen present in the T-lymphocytes.

Bursa fabricus:-

This is a sac like lymphoepithelial structure. This organ was first described in 1621 by fabricus, an italian scientist and hence the name this organ originates from the hind gut epithelium of the chick embryo at about the 15th day of development. By the time of hatching it developed into a fully functioning organ. By about four months of age, it reaches the maximum size of about 3 cm in diameter. After which starts of involving the structure of bursa is very similar to that of thymus. It begins to develop and epithelial structure and later becomes colonized by lymphoid cells arising from the foetal liver and bone marrow.

A cross section of bursa fabricus shows that the structure is very similarly to thymus. It is divided into follicles with in there are different types of cells.

Among these cells, the most prominent are lymphocytes, macrophages and plasma cells. Bursa cell proliferation give rise to the cortex and the medulla in bursa follicle as the thymus. The lymphocytes of bursa are similar to the lymphocytes of the thymus in morphology but differ significantly in function.


Bone marrow:-

Bone marrow is the soft tissue with in the cavities of bones. In mammals it is the largest tissue which a total weight of about 3 kgs an average human adult. 

Bone marrow can be divided into two regions namely–

  • Yellow bone marrow
  • Red bone marrow

The yellow bone marrow nutrients and remove waste actively growing blood cells this tissue forms about half of the weight of the bone marrow. The other half of the bone marrow which is actively involved in haemopoisis is known as red bone marrow. Please contain all the different cells of blood namely erythrocytes, platelets, granulocytes, monocytes and lymphocytes with their precursor. In adult much of the red bone marrow is replaced by the fatty tissue and become yellow bone marrow.

The lymphoid stem cells of the bone marrow from into t and B-cells. The t cells enters into when the differentiate into mature T-lymphocytes which involved in cell mediated immune response. The B-cells undergo differentiation in the bone marrow itself and thus bone marrow acts as a primary lymphoid organ.


Secondary lymphoid organs : –

Lymph node and spleen are the most highly organized of secondary lymphoid organs. Less organized lymphoid tissue, collectively called mucosal associated lymphoid tissue { M A L T} occurs in various body sites. Malt { mucosal associated lymphoid tissue} includes payers patches, tonsils, appendix.


Lymph nodes are encapsulated bean shaped structures containing a reticular packet network with lymphocytes, macrophages and dendritic cells. Lymphnodes are the first organised lymphoid structure to encounter antigens that enters the tissue spaces. Any antigen entered into the lymphnode are trapped by the cellular network. The overall architectures of lymphocytes to effectively encounter the trapped antigen.

A lymphnode morphologically divided into 3 concentric regions:-

  1. The cortex
  2. Paracortex
  3. Medulla

The outermost layer, the cortex contains lymphocytes { mostly B-cells}, macrophages and dendritic cells – following antigenic challenge, b cell activation, differentiation occurs and give plasma and memory B-cells.

We need to the cortex is the para cortex which has large number of t cells and dendritic

Cells. These dendritic cells express II mhc molecules for antigen presentation to th cells. Hence called thymus dependent area.

The innermost layer of a lymphnode, the medulla contains lymphocytes but many of these are antibody molecules.



The spleen is large, ovoid secondary lymphoid organ situated high in the abdominal cavity. The spleen adapted filtering blood and trapping blood borne pathogens { antigens}. The spleen is surrounded by a capsule and has lobes which are divided by “trabeculae”.

The compartments are two types:-

The red pulp

The white pulp

 The red pulp:-

The red pulp consists of network of siusoids with macrophages and red blood cells {Rbc}.


The white pulp:-

The white pulp surrounded by the arteries forming peri anterior lymphoid sheath { P L A S } populated by many T-lymphocytes.

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