Wednesday, November 9, 2011

The basics of immunology

Immunology scares me. I'm not ashamed to admit this fact. I find the topic intimidating and overwhelming, especially when I listen to talks given by prominent immunologists. The terminology is difficult, and the concepts seem very intertwined. I've always perceived that breaking into understanding immunology required a lot of work but that it would (and should) make sense... eventually.

 The next few blog posts are going to focus on immunology, not only because I need to learn this information, but also because it is fascinating and a challenging topic.

Components of the Immune System
All of the cells that comprise the immune system emerge from the bone marrow, where all of them originally come from and where some of them remain for maturation.  The cell type that gives rise to immune cells is the hematopoietic stem cell.  From this pluripotent state, the hematopoetic stem cell can then mature into a myeloid progenitor cell or a common lymphoid progenitor.  Myeloid progenitor cells can differentiate into several more cell types, including granulocyte and macrophage progenitors and megakaryocyte and erythrocyte progenitors.  The granulocyte and macrophage progenitors can then develop into neutrophils, eosinophils, basophils, mast cells, and macrophages.  Megakaryocyte and erythrocyte progenitors generate platelets upon maturation.

Hematopoietic stem cells can also develop into a common lymphoid progenitor, which consists of B cells, T cells, and NK cells.  These types of cells leave the bone marrow and migrate through the lymph nodes.  Dendritic cells also develop from lymphoid progenitor cells but mature in the bone marrow before entering the lymph node.

Basic functions of immune cells
  • Macrophages are a common cell type that mature from monocytes (from the myeloid progenitor cells originally).  Monocytes circulate in the blood and continuously differentiate into macrophages when they enter the body's tissues.  Once in the tissues, macrophages can be considered the garge trucks of the body:  they engulf the environment as well as other cells in the process of phagocytosis.  Thus, macrophages can function to neutralize harmful elements within the body.
  • Dendritic cells also mature from myeloid progenitor cells, and their main function is to process and display antigen that will then be readable by T lymphocytes.  This antigen display requires the presentation of co-stimulatory molecules, and when dendritic cells encounter a pathogen (or other foreign antigen), they mature and begin expressing these co-stimulatory molecules.
  • Mast cells differentiate in body tissues and are involved in mediating mucosal immunity.  They are most well-known for their role in allergic reactions.
  • Neutrophils are a type of granulocyte (so called because they have densely-staining and strange-shaped nuclei) that are involved in phagocytosis and increase in numbers upon an immune response.
  • Eosinophils respond to parasites.
  • Basophils may function similarly to mast cells.
  • B cells differentiate into plasma cells and function to secrete antibodies.
  • T cells destroy virus-infected cells and also function to activate other immune cells, such as B cells and macrophages.
  • NK cells are involved in innate immunity and destroy "weird-looking" cells, such as tumor cells or cells infected with viruses.
References for the interested:
Immunobiology. Janeway, Travers, Walport, Shlomchik.
Basic Concepts of Immunology and Neuroimmunology: Basic Immunology

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