The Cells of the Immune System

The response to pathogens is orchestrated by the complex interactions and activities of the large number of diverse cell types involved in the immune response. The innate immune response is the first line of defense and occurs soon after pathogen exposure. It is carried out by phagocytic cells such as neutrophils and macrophages, cytotoxic natural killer (NK) cells, and granulocytes. The subsequent adaptive immune response includes antigen-specific defense mechanisms and may take days to develop. Cell types with critical roles in adaptive immunity are antigen-presenting cells including macrophages and dendritic cells. Antigen-dependent stimulation of various cell types including T cell subsets, B cells, and macrophages all play critical roles in host defense. [1]

How are these cells formed?

Hematopoiesis

Hematopoiesis is the process by which immature precursor cells develop into mature blood cells.  The currently accepted theory on how this process works is called the monophyletic theory which simply means that a single type of stem cell gives rise to all the mature blood cells in the body.  This stem cell is called the pluripotential (pluripotent) stem cell [2]

SITES OF HEMATOPOIESIS [2]

The Process:

Early in hematopoiesis, a multipotent stem cell differentiates along one of two pathways, giving rise to either a common lymphoid progenitor cell or a common myeloid progenitor cell. The types and amounts of growth factors in the microenvironment of a particular stem cell or progenitor cell control its differentiation. During the development of the lymphoid and myeloid lineages, stem cells differentiate into progenitor cells, which have lost the  capacity for self-renewal and are committed to a particular cell lineage. Common lymphoid progenitor cells give rise to B, T, and NK (natural killer) cells and some dendritic cells. Myeloid stem cells generate progenitors of red blood cells (erythrocytes), many of the various white blood cells (neutrophils, eosinophils, basophils, monocytes, mast cells, dendritic cells), and platelets.Progenitor commitment depends on the acquisition of responsiveness to particular growth factors and cytokines. When the appropriate factors and cytokines are present, progenitor cells proliferate and differentiate into the corresponding cell type, either a mature erythrocyte, a particular type of leukocyte, or a platelet-generating cell (the megakaryocyte). Red and white blood cells pass into bonemarrow channels, from which they enter the circulation. [3]

In bone marrow, hematopoietic cells grow and mature on a meshwork of stromal cells, which are nonhematopoietic cells that support the growth and differentiation of hematopoietic cells. Stromal cells include fat cells, endothelial cells, fibroblasts, and macrophages. Stromal cells influence the differentiation of hematopoietic stem cells by providing a hematopoietic-inducing microenvironment (HIM) consisting of a cellular matrix and factors that promote growth and differentiation. Many of these hematopoietic growth factors are soluble agents that arrive at their target cells by diffusion, others are membrane-bound molecules on the surface of stromal cells that require cell-to-cell contact between the responding cells and the stromal cells. During infection, hematopoiesis is stimulated by the production of hematopoietic growth factors by activated macrophages and T cells. [3]

Here is the flow chart of the same for easier understanding;

[4] Source: Hematopoietic Stem Cells, Treating Lymphoma and Leukemia with Hematopoietic Stem Cells, By: Madison Altendorf, from https://microbiologyspring2011.wikispaces.com/Hematopoietic+Stem+Cells

To understand this flowchart better, here have a look at the vidoe from Handwritten Tutorials by visiting our video gallery and view "cells of the immune system" or 

Click here:  https://www.youtube.com/watch?v=Bn5Rw16buSA

So it all starts from the Stem cells, here is more information about these cells of our life

Click here :   https://microbiologyspring2011.wikispaces.com/Hematopoietic+Stem+Cells

References      

[1] Research Topics, Cells of the Immune System from https://www.rndsystems.com

[2] https://instruction.cvhs.okstate.edu/histology/HistologyReference/hrhemac.htm

[3] Chapter 2, Immunology, Kuby, Fifth Edition.

[4]  Hematopoietic Stem Cells, Treating Lymphoma and Leukemia with Hematopoietic Stem Cells, By: Madison Altendorf, from https://microbiologyspring2011.wikispaces.com/Hematopoietic+Stem+Cells