The human immune system is made up of a number of interdependent cell types which collectively protect the person's body from various parasitic, fungal, bacterial and viral infections, as well as from the growth of tumor cells.
A number of these cell types have specialized functions, are able to kill parasites, engulf bacteria, or kill tumor cells or viral-infected cells. Frequently, these cells are dependent upon the, 'T,' helper subset for activation signals in the form of secretions which are more formally referred to as, 'Lymphokines,' 'Cytokines,' or specifically as, 'Interleukins.' An understanding of the T helper subset may assist in comprehension of the root of immune deficiencies, as well as perception of the potential avenues that the human immune system can be modulated in the case of particular diseases.
Immune System Organs
Bone Marrow: Every cell involved in a person's immune system is initially derived from bone marrow. These cells form through a process referred to as, 'Hematopoiesis.' During hematopoiesis bone marrow derived stem cells differentiate into one of two things; either mature cells of the immune system, or precursors of cells which then migrate out of the person's bone marrow, continuing their maturation elsewhere in the body. Bone marrow produces, 'B,' cells, killer cells, immature thymocytes, and granulocytes, as well as platelets and red blood cells.
Thymus: The Thymus' function is to produce mature, 'T,' cells. Immature Tymocytes, also referred to as, 'Prothymocytes,' emerge from the person's bone marrow and move into their thymus. Through a process called, 'Thymic Education,' these T cells which are beneficial to the person's immune system are spared while T cells that may cause a detrimental autoimmune response are removed. Mature T cells are released into the person's blood stream.
Spleen: A person's spleen is an immunological filter, filtering their blood. The spleen is comprised of T cells, B cells dendritic cells, macrophages, red blood cells and natural killer cells. Macrophages and dendritic cells not only capture foreign materials called, 'Antigens,' from a person's blood which passes through the spleen, they bring these antigens to the spleen itself from the person's blood stream. A person's body experiences an immune response when the macrophage or dendritic cells present the antigen to appropriate T or B cells. In a person's spleen, B cells are activated and produce great amounts of antibody. The spleen also destroys old red blood cells.
Lymph Nodes: A person's lymph nodes work as an immunological filter for their bodily fluid referred to as, 'Lymph.' People have lymph nodes throughout their body. Lymph nodes are made mostly of B cells, T cells, macrophages, and dendritic cells. A person's lymph nodes drain fluid from the majority of their tissues. Lymph nodes filter out antigens from lymph prior to returning lymph to the person's body for circulation. In a manner much like the spleen, dendritic cells and macrophages that capture antigens present foreign materials to both B and T cells, initiating an immune response.
Immune System Cells
T Cells: T Lymphocytes are commonly placed into two major subsets which are identifiably different. One of these subsets is the, 'T Helper Subset,' also referred to as the, 'CD4+ T Cell,' which is a coordinator of a person's immune regulation. The primary function of the T helper cell is augmentation of the person's immune responses through secretion of specialized factors which activate additional white blood cells in order to fight off an infection.
CD8+ T Cells: CD8+ T Cells are also called T killer/suppressor cells and are important because they are involved in directly killing viral infected cells, specific tumor cells, and sometimes parasites. CD8+ T cells are important because they also down-regulate immune responses. While both types of T cells are found throughout a person's body, they are many times dependent on the lymph nodes and spleen as places where activation happens, yet are also found in other tissues in a person's body, notably the person's lungs, liver, blood and intestinal and reproductive tracts.
Natural Killer Cells: Natural killer cells are also called, 'NK Cells,' and are similar to cells from the killer T cell subset. Natural killer cells work as effector cells, killing specific tumors like lymphomas, melanomas, and viral infected cells such as herpes and cytomegalovirus infected cells. Natural killer cells kill their targets in the person's lymphoid organs; however, these cells which have been activated through secretions from CD4+ T cells will kill viral-infected targets or tumors more efficiently.
B Cells: The primary function of B cells is to produce antibodies in response to foreign proteins such as viruses, bacteria and tumor cells. Antibodies are proteins that specifically first recognize and then bind to one other particular protein which also specifically recognize and bind to one particular protein. The production of antibodies and binding to foreign substances or antigens is often critical as a means of signaling additional cells to engulf and kill, or remove a substance from the person's body.
Polymorphonuclear (PMN) Leukocytes or Granulocytes: There is a group of white blood cells which is collectively referred to by medical personnel as, 'Polymorphonuclear Leukocytes (PMN's),' or, 'Granulocytes.' Granulocytes are made of three cell types which are referred to as, 'Eosinophils,' 'Neutrophils,' and, 'Basophils.' The names for them are based on their staining characteristics with specific dyes. The cells are important because they are involved in the removal of parasites and bacteria from a person's body. These cells first engulf foreign bodies and then degrade them by using their powerful enzymes.
Macrophages: Macrophages are important due to their function in the regulation of immune responses. Macrophages are many times referred to as, 'scavengers,' or, 'Antigen-Presenting Cells (APC's),' because they both pick up and ingest foreign materials, and then present these antigens to other cells in a person's immune system like B cells and T cells. The process is one of the steps involved in initiating an immune response. Microphages that have been stimulated show increased levels of, 'Phagocytosis,' and are also secretory.
Dendritic Cells: Dendritic cells also originate in a person's bone marrow, working as antigen presenting cells (APC's). Interestingly, dendritic cells are more efficient APC's than macrophages. Dendritic cells are commonly found in the structural compartment of a person's lymphoid organs, such as their spleen, lymph nodes and thymus. These cells can also be found in the person's blood stream, as well as in other tissues in their body. Science believes that these cells capture antigen, or bring it to the person's lymphoid organs, where an immune response is initiated. One of the reasons that scientists know very little about dendritic cells is because they are difficult to isolate; something that is often a prerequisite for studying the functional qualities of exact cell types. Of particular note is a recent finding that dendritic cells bind high amount of HIV and could be a reservoir of virus which is transmitted to CD4+ T cells during and activation event.
The presence of an APC, combined with a T cell or B cell, is required in order for there to be an immune response to a foreign antigen. Should an APC present an antigen on it's cell surface to a B cell, for example, the B cell is signaled to proliferate and produce antibodies. The antibodies then specifically bind to that antigen. If the antibodies bind to antigens on parasites or bacteria, it acts as a signal for macrophages or PMN's to engulf and kill them. One addition and important function of antibodies is to start something referred to as a, 'Complement Destruction Cascade.' When antibodies bind to bacteria or cells, serum proteins referred to as, 'Complement,' first bind to immobilized antibodies, and then destroy the bacteria through creating holes in the bacteria. Antibodies may also signal macrophages and natural killer cells to kill bacterial infected cells or viral cells.
Should the APC present an antigen to T cells, the T cells activate, proliferate, and become secretory in the case of CD4+ cells or - if they are CD8+ cells, they become active and kill target cells that specifically express the antigen presented by the APC. Production of antibodies, as well as activity of CD8+ killer T cells, is greatly regulated by the CD4+ helper T cell subset. These CD4+ T cells provide signals to these cells which tell them to both function and proliferate with increased efficiency. The multitude of cytokines or interleukins which are produced and secreted by CD4+ T cells are many times crucial in order to ensure the activation of macrophages, CD8+ T cells, natural killer cells, and PMN's.
Also see a simplified version of this article in The Human Immune System Part 2