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All the cells of the immune system inclusive of blood cells originate from the bone marrow during haematopoiesis from progenitor haematopoietic stem cells(Figure 1). Haematopoiesis is simply defined as the biological process involved in the formation of blood cells (i.e., white blood cells, red blood cells and platelets) and other cellular components of blood particularly the immune system cells such as the B cells and T cells in the bone marrow. During haematopoiesis, progenitor haematopoietic stem cells in the medullar of the bone or bone marrow proliferate into progenitor erythroid cells, myeloid cells and lymphoid cells. The erythroid cells proliferate into erythrocytes or oxygen-carrying red blood cells (RBCs) and reticulocytes (i.e., immature RBCs) while the myeloid cells proliferate into myelocytes or myelocyte cells (e.g., macrophages, granulocytes and megakaryocytes) which play critical roles in both adaptive immune response and innate immune response. The lymphoid cells proliferate mainly into lymphocytes which include T cells and B cells or white blood cells (WBCs).

Figure 1. An overview of the development of cells that take part in immune response. Photo courtesy: Online Textbook of Bacteriology by Dr. Kenneth Todar, Department of Bacteriology University of Wisconsin, USA. 

The immune system is a complex system or network that comprises several cells and organs and other immunological products (e.g., antibodies) which work cooperatively to protect the body from infectious diseases and infections. The immune system also protects the body from disease-causing pathogenic microorganisms including bacteria, fungi, protozoan and viruses, and from other harmful substances or chemicals that penetrate the body. Every living animal (inclusive of humans) are inundated and fashioned with these complex networks of cells (i.e., immune system) and organs that help them to fight against the invasion and colonization of the body by pathogenic microorganisms or antigens. These immune system cells are found dispersed in every part of the body; and they mount immunological actions to counteract the negative consequences of invading antigens or pathogens. The immune system is man’s natural mechanism of keeping infectious diseases at bay; and when the immune system is compromised (e.g., through malnutrition, stress and the presence of a debilitating disease like HIV/AIDS), the individual becomes exposed to plethora of infectious disease agents that invades the body, colonizes it and causes disease/infections.

Thus, the effectiveness of the immune system of the host is critical to the prevention of invasion and colonization of the body by pathogenic microorganisms. The cells of the immune system include lymphoid cells, lymphocytes, phagocytes, macrophages, dendritic cells, memory B cells, plasma cells, granulocytes, megakaryocytes, basophils, eosinophils, neutrophils, null cells and monocytes. These cells become mobilized to respond to any invading antigen or pathogen that threatens the normal physiological conditions of the host organism including humans and animals.

Further reading

William E.P (2003). Fundamental Immunology. 5th edition. Lippincott Williams and Wilkins Publishers, USA.

Stevens, Christine Dorresteyn (2010). Clinical immunology and serology. Third edition. F.A. Davis Company, Philadelphia.

Silverstein A.M (1999). The history of immunology. In Paul, WE (ed): Fundamental Immunology, 4th edition. Lippincott Williams and Wilkins, Philadelphia, USA.

Paul W.E (2014). Fundamental Immunology. Seventh edition. Lippincott Williams and Wilkins, USA.

Male D, Brostoff J, Roth D.B and Roitt I (2014). Immunology. Eight edition. Elsevier Saunders, USA.

Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.

Berzofsky J.A and Berkower J.J (1999). Immunogenicity and antigen structure. In Fundamental Immunology, 4th edition., W.E. Paul, ed., Lippincott-Raven, Philadelphia. 

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