Cells involved in Immunity & their role

Jonathan Ausman



Hemopoeitic Stem Cell:
The hemopoeitic stem cell is a pluripotent cell with the capability of differentiating into all of the cells in the circulating blood. A proportion of these hemopoetic stem cells remain in their original form in the bone marrow as a constant supply to the process of differentiation. The numbers of pluripotent hemopoetic stem cells diminish with age.

1. Myeloid Progenator:
These cells give rise to the lineage of granulocytes and monocytes. These cells are formed primarily in the bone marrow.

a. Granulocytes
The granulocytes are a group of cells that are defined by their microscopic granular appearance. Though these cells share their characteristic of granular cytoplasm and multiple nuclei, the three groups in this category have differing functions:

i. Neutrophils: These granulocytic cells are highly phagocytotic, especially towards bacteria and are the first cells to accumulate in the site of acute inflammation.
ii. Eosinophils: These granulocytes are mediators of allergic reactions as they have a neutralizing effect on the inflammation inducing substances released by other cells. The other main function of Eosinophils is in the defense against parasites.
iii. Basophils: These granulocytes are responsible for allergic reactions and mediate their effect through the action of Immunoglobulin E (IgE). These cells work with mast cells to release histamine, bradykinin, serotonin, heparin and lysosomal enzymes.

b. Monocytes and Marcrophages:
The Monocytes are white blood cells characterized by one single nucleus and are the parent cells to the Macrophages. The monocytes are stem cells for the formation of Marcrophages which are the highly phagocytotic cells found in the tissues and which are recruited largely in the case of chronic inflammation.

2. Lymphoid Progenator:
a. These cells are the parent cells in the lymphocytic lineage and give rise to the cells known as lymphocytes. The production of these cells occurs in the lymphogenous tissues of the body such as the spleen, thymus, lymph glands and tonsils.
b.
i. T-Lymphocytes:
There are three groups of T-Lymphocytes that have various functions:
1. Helper T-Cells:[/color] These T-Lymphocytes have the role of initiating many mediators in the immune system. These cells release Lymphokines which stimulate the growth and development of the other T-lymphocytes, cause the growth and differentiation of B-Lymphocytes into plasma cells and antibodies, release interleukins which are large mediators in inflammation and activate the macrophage system.
2. Cytotoxic T-Cells: These T-lymphocytes are responsible for the attack of foreign microorganisms and in some cases the body’s own tissue. These cells act by punching holes in the membrane of the attacked cell through the action of perforins; also these cells release cytotoxic substances to complete the task of killing unwanted cells before moving on to attack others.
3. Suppressor T-Cells: these T-Lymphocytes are responsible for suppression of the other two types of T-lymphocytes and are important in limiting autoimmune disorders as Cytotoxic T-Lymphocytes have the ability to attack the body’s own tissues.

ii. B-Lymphocytes and Plasma cells:
B-Lymphocytes have the action of receiving the foreign antigen from the adjacent macrophages. The B-Lymphocytes rapidly reproduce and mature to form plasma cells. These plasma cells produce the antibodies required to combat the antigen. A proportion of the original B-Lymphocytes will carry the antigen to the lymphoid tissue in the body and act as “memory” cells to enhance the speed of antibody response should the same antigen reoccur

*Dendritic cells, much like the macrophages and B-Lymphocytes have an antigen presenting characteristic. Dendritic cells are the most potent of these cells at this function and their only known function is to present the antigens to the T-Cells.
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3. Megakaryocyte and Platelets:
The megakaryocytes are cells that resemble the white blood cells in the bone marrow. These cells undergo fragmentation and form platelets, which are required in initiation of blood clotting during vascular endothelial damage in the human body.