Cells of Immune system

This category of cells includes various elements characterized by the ability to engulf or "eat" agents or materials potentially harmful to the organism. The macrophages are the kind of absorbs more prevalent in body tissues and are derived from phagocytes that circulate in the blood called monocytes. In some organs, phagocytes take on particular names such as Kupffer cells, in the liver and microglia cells, in the central nervous system. Phagocytes develop in the bone marrow; their functioning is regulated by cytokines, and, in turn, phagocytes can produce cytokines to transmit stimuli to other cells of the immune system. Phagocytes are the body's first line of defense, and similar cells play the same role in very basic organisms. 

The phagocytes play the main three functions. They transfer microorganisms, structures and molecules inside themselves, through phagocytosis, which, left in the blood or tissues, could create problems. For example, macrophages engulf viruses or portions of bacteria, but also cholesterol that circulates in the blood linked to proteins (lipoproteins). Once engulfed, these materials are "digested" by the cells, and some parts or molecules of the engulfed structures are selected as antigens. Phagocytes can perform a second function, which is to expose the selected molecule as an antigen on its membrane, and for this reason, they are also called “antigen-presenting cells” (Antigen Presenting Cell: APC). 

The exposure of the antigen on the membrane serves to activate a response towards the antigen or the organism. This way, it derives, which will be dedicated to the different types of immune responses. The third function of phagocytes is the synthesis and release of cytokines such as interleukins 1 (IL-1), 6 (IL-6), 8 (IL-8), and 12 (IL-12) and the tumor necrosis factor-alpha (Tumor Necrosis Factor-alpha: TNFα). They are used to coordinate the activity of phagocytes with the other mechanisms of the immune system.

Granulocytes

They are cells of the immune system that owe their name to the presence within them of "granules" of various shapes and which present different colors when placed in contact with specific dyes. The granulocytes neutrophils and eosinophils have the ability to bind and destroy potentially harmful agents such as bacteria. The eosinophilic granulocytes and those defined basophils express molecules on the surface of the membrane called receptors that bind the antibodies of the type IgE. Basophilic granulocytes are the circulating version of cells called mast cells in the blood present in the tissues. The granules of basophilic granulocytes contain molecules and mediators, such as histamine. They are involved in both the mechanisms of inflammation and those of allergic reactions.

Lymphocytes

The definition of lymphocytes includes various series of cells that perform different types of functions within the mechanisms of the immune response, but all lymphocytes have in common the characteristic of responding to specific aggressions. And It differentiates them, for example, from phagocytes that do not have specific goals in their action. The ones listed below are the main types of lymphocytes.

Natural Killer Lymphocytes (NK)

Lymphocytes Natural Killer is a type of lymphocyte that can recognize cells infected by virus or transformed into cells of tumors and eliminate them. It is evident that the function of these cells is very delicate. Identifying, among many cells of the same type, the infected or degenerated one can be very difficult. NK lymphocytes do this by referring to particular molecular structures found on the outer surface (membrane) of such cells called the Major Histocompatibility Complex. In particular, the one involved in this mechanism is class I MHC. The activity of NK lymphocytes is enhanced by cytokines such as gamma interferon (IFgamma) and interleukin 2 (IL-2).

T lymphocytes

T lymphocytes derive from cells that develop in the bone marrow and then pass into the thymus where they mature and differentiate into cell subtypes. In particular, there are T lymphocytes defined as cytotoxic (T lymphocytes c or CTLs) to mean that they are capable of destroying cells with which they come into contact. And, in particular, those infected with viruses or degenerated into tumor cells. Furthermore, the cytotoxic T lymphocytes release cytokines, which stimulate the activity of phagocytes. Another type of T lymphocyte is called a helper (h T lymphocytes). 

These lymphocytes have the main function of releasing cytokines, which, in turn, regulate the production and differentiation of other T lymphocytes and also of phagocytes and B lymphocytes. Among the cytokines produced, there are interleukin 2 (IL- 2), which increases the production of T lymphocytes, interleukin 4 (IL-4). It stimulates the development of B lymphocytes and other interleukins and interferon-gamma (IFNγ), which contribute to the regulation of the immune response. T helper lymphocytes are classified into subtypes according to the type of cytokine they produce (T h1, T h2, T h3, etc.). An additional type of lymphocyte is called memory lymphocytes (Tm lymphocytes) and has the characteristic of preserving the memory of the contact between an individual's immune system and the antigen. 

Finally, there are lymphocytes called regulators (Treg lymphocytes), which are decisive in preventing the immune system from attacking molecules that are not harmful to the body. Precisely for this inhibiting effect of some responses of the immune system, suppressor T lymphocytes have been called regulatory T lymphocytes. They are partly produced in the thymus and partly formed during an immune system response, precisely to prevent it from improperly attacking non-aggressive molecules for the body. 

A characteristic common to all T lymphocytes, and which distinguishes them from NK lymphocytes and B lymphocytes, is that they present proteins on the external membrane that are crucial for the development of their functions. These proteins are indicated by the abbreviation CD accompanied by a number. For example, T helper lymphocytes present a protein on the membrane called CD4, while the one present on cytotoxic T lymphocytes is called CD8. For this reason, these cells are also referred to as CD8 + T lymphocytes. Memory T lymphocytes can have both CD4 and CD8 proteins on the membrane.

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Author: Vicki Lezama