Biology Notes on Immune System | Immunology

The below mentioned article provides notes on immune system.

Immune Response:

Any foreign protein, toxin of parasites, bacte­ria and viruses, when enter into the body, they interfere with host physiological processes and produce harmful effects. The “Chemical defense” mechanism of host organism that operates against such effects of parasites and others is called immune reaction or immune response.

Immunology:

The branch of life science which deals with immune reaction is known as immunology.

Components of Immune System:

The immune system consists of a network of diverse organs and tissue which vary structurally as well as functionally from each other. These organs remain spreaded throughout the body. Basically, immune system is a complex network of lymphoid organs, tissues and cells.

These lym­phoid organs can be categorized under three types depending upon their functional aspects:

i. Primary lymphoid organ.

ii. Secondary lymphoid organ.

iii. Tertiary lymphoid organ.

White blood cells or leukocytes are the basic cell types which help to give rise to different types of cells which participate in the development of immune response (Fig. 2.1). WBC are classified into granulocytes and agranulocytes depending on the presence or absence of granules in the cyto­plasm.

Agranular leukocytes are of two types, viz., lymphocytes and monocytes. Lymphocytes play pivotal role in producing defensive molecules of immune system. Out of all leukocytes, only lymphocytes possess the quality of diversity, specificity, memory and self-non self recognition as various important aspects of immune response.

Other cell types remain as accessory one; help to activate lymphocytes, to generate various immune effector cells, to increase the rate of anti­gen clearance etc. (Fig. 2.2).

Hematopoiesis:

Mature blood cells have a relatively short span of life and consequently the population must be continuously replaced by the progeny of stem cells produced in the hemopoietic (Gr. haema, blood; poiesis, a making) organs.

Initiation of Hematopoiesis:

Hematopoiesis begins in the yolk sac in the first week of embryonic development. Here yolk-sac stem cells differentiate into primitive erythroid cells containing embryonic haemoglobin. In the third month of gestation, the stem cells migrate from the yolk sac to the foetal liver and then to the spleen; these two organs have major roles in hematopoiesis from third to the seven month of gestation. As gestation continues, the bone marrow becomes the major hemopoietic organ. By birth hematopoiesis has ceased within the liver and spleen.

Stem cells, Growth factors and Differen­tiation:

It is thought that all blood cells arise from a single type of stem cell is the bone marrow. Bone marrow can produce all blood cell types. It is called a pluripotential or pluripotent stem cell. This cell proliferates and differentiates and giving rise to lymphoid and myeloid stem cells. Both of these two types of stem cells are called multi- potential stem cells.

Subsequent differentiation of lymphoid and myeloid stem cells generate committed progeni­tor cells for each type of mature blood cells. These progenitor cells or progenitor stem cells have lost the capacity of self-renewal and are committed to a particular cell lineage.

Progenitor commitment depends on the acqui­sition of responsiveness to particular growth fac­tors, when appropriate growth factors are present. These progenitor cells proliferate and differen­tiate, giving rise to the corresponding type of mature red or white blood cells.

In adult bone marrow, the hemopoietic cells grow and mature on a meshwork of stromal cells, (fat cells, endothelial cells, fibroblasts and macrophages) which are non-hematopoietic in nature. Stromal cells influence the hematopoietic stem cell differentiation by providing a hemo­poietic inducing microenvironment consisting of a cellular matrix and factors that promote growth and differentiation.

As hematopoietic stems cells differentiate in this microenvironment, their membranes acquire deformability, allowing the mature cells to pass through the sinusoidal wall into the sinuses of the bone marrow from where they enter the circulation.

Hematopoietic growth factors:

Various growth factors have been shown to be required for the survival, proliferation, differentia­tion and maturation of hemapoietic cells in culture. Originally these growth factors or a host of cytokines [proteins made cells that affect the behaviour (growth, function) of other cells; a medi­ator of communication between the cells) were detected in serum or in conditioned medium from in vitro (outside the living system) cell-cultures.

Effect of Cytokines on Haematopoietic Cells:

They subsequently were defined on the basis of their ability to stimulate the formation of hemo­poietic cell colonies in bone marrow cultures. Among the cytokines, there was a family of acidic glycoproteins, the colony stimulating fac­tors (CSFs), the name given for their ability to induce the formation of distinct hemopoietic cell lineages. These CSFs act in a stepwise manner including proper maturation of hemopoietic cells.

Classes of colony stimulting factors (CSFs):

CSFs can be classified under four distinct cate­gories:

1. Multi-lineage stimulating factor:

Multi-CSF is also known as interleukin 3(IL-3).

This group acts during early differentiation, induces the formation of all non-lymphoid cells including erythrocyte, monocytes and granulo­cyte.

2. Granulocyte-macrophage colony stimu­lating factor (Gm-CSF):

GM-CSF acts slightly in later stage but also induces the formation of non-lymphoid cells.

3. Macrophage-colony-stimulating factor (M-CSF).

4. Granulocyte-colony-stimulating factor (G-CSF).

These two groups help to promote the forma­tion of neutrophil and monocyte.

Besides these four groups, some other impor­tant cytokines take part in the hematopoiesis.

These are:

(i) Erythropoietin, this cytokines induces terminal erythrocyte development, regu­lates red blood cells production;

(ii) lnterleukin-4 (IL-4)—promotes T_H2 differentiation;

(iii) IL-5— induces eosinophil activation and generation;

(iv) IL-6—influences adaptive immunity;

(v) IL-7— the main function is T-cell survival;

(vi) IL-8— induces the neutrophil to extravasate into the tissues;

(vii) IL-9—it is a third signal transducing subunit.

Control/Regulation of Hematopoiesis:

Haematopoiesis is a continuous process which a steady state of the production and loss of red and white blood cells. Haematopoiesis is regu­lated by complex mechanism that affect all of the individual cell type.

The overall regulatory mechanisms are repre­sented below schematically:

Cells of the Immune System:

The central cell of the immune system is the lymphocyte, which account for roughly 25% of the white blood cells in blood and 99% of the cells in the lymph. A variety of white blood cells or leukocytes, participate in the development of an immune response.

Of these cells, only the lymphocytes possess the attributes of diversity, specificity, memory and self/non-self recog­nition, the hallmark of an immune response. All other cells play accessory roles, serving to activate lymphocytes, to increase the effectiveness of antigen clearance by phagocytosis or to secrete vari­ous immune effector molecules. During hematopoiesis, four major cell linea­ges arise from the HSC’s.

Origin of the Cells of the Immune System: