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Which Cells Are Important Components Of The Human Immune System

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White blood cells, also known as leukocytes or white corpuscles, are a cellular component of blood that does not have hemoglobin, has a nucleus, is able to move, and protects the body against infection and disease. White blood cells carry out their protective activities by engulfing foreign materials and cellular debris, by destroying infectious agents and cancer cells, or by producing antibodies. Although white cells are found in the circulation, most appear outside the circulation, in the tissues where they fight infections; the few in the bloodstream pass from one place to another. White cells are highly differentiated due to their specialized functions and do not undergo cell division (mitosis) in the bloodstream; however, some retain the ability to undergo mitosis.

Based on their appearance under a light microscope, white cells are grouped into three main classes—lymphocytes, granulocytes, and monocytes—each of which performs somewhat different functions. Lymphocytes, which further divide into B cells and T cells, are responsible for the specific recognition of foreign agents and their subsequent removal from the host. Granulocytes, the most numerous of the white cells, rid the body of large pathogenic organisms such as protozoa or helminths and are also key mediators of allergy and other forms of inflammation. Monocytes, which represent between 4 and 8 percent of the total number of white blood cells in the blood, travel from the blood to sites of infection, where they further differentiate into macrophages.

A healthy adult has between 4,500 and 11,000 white blood cells per cubic millimeter of blood. Fluctuations in the number of white blood cells are observed during the day; lower values ​​are obtained during rest and higher during exercise. An abnormal increase in the number of white cells is known as leukocytosis, while an abnormal decrease in the number is known as leukopenia. The number of white blood cells can increase in response to intense physical exertion, convulsions, acute emotional reactions, pain, pregnancy, childbirth, and certain disease states, such as infections and intoxications. The number may decrease in response to certain types of infections or drugs, or in association with certain conditions, such as chronic anemia, malnutrition, or anaphylaxis. In general, newborns have a high white blood cell count that gradually falls to the adult level during childhood.

White blood cells, a cellular component of blood that lacks hemoglobin, has a nucleus, is able to move, and defends the body against infection and disease by engulfing foreign materials and cellular debris, by destroying infectious agents and cancer cells, or by producing antibodies .

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In adults, the bone marrow produces 60 to 70 percent of the white cells (ie, granulocytes). Lymphatic tissues, especially the thymus, spleen, and lymph nodes, produce lymphocytes (comprising 20 to 30 percent of white cells). The reticuloendothelial tissues of the spleen, liver, lymph nodes, and other organs produce monocytes (4 to 8 percent of white cells). A healthy adult has between 4,500 and 11,000 white blood cells per cubic millimeter of blood. Fluctuations in the number of white blood cells are observed during the day; lower values ​​are obtained during rest and higher during exercise.

The survival of white blood cells, as living cells, depends on their continuous production of energy. The chemical pathways used are more complex than those of red blood cells and are similar to those of other tissue cells. White cells containing a nucleus and capable of producing ribonucleic acid (RNA) can synthesize protein.

Two neutrophils among many red blood cells. Normal neutrophil functions are compromised in chronic granulomatous disease.

Although white cells are found in the circulation, most appear outside the circulation, in the tissues where they fight infections; the few in the bloodstream pass from one place to another. As living cells, their survival depends on their continuous production of energy. The chemical pathways used are more complex than those of red blood cells and are similar to those of other tissue cells. White cells containing a nucleus and capable of producing ribonucleic acid (RNA) can synthesize protein. White cells are highly differentiated due to their specialized functions and do not undergo cell division (mitosis) in the bloodstream; however, some retain the ability to undergo mitosis. Based on their appearance under a light microscope, white cells are grouped into three main classes—lymphocytes, granulocytes, and monocytes—each of which performs somewhat different functions.

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Lymphocytes, which further divide into B cells and T cells, are responsible for the specific recognition of foreign agents and their subsequent removal from the host. B-lymphocytes secrete antibodies, which are proteins that bind to foreign microorganisms in body tissues and mediate their destruction. Normally, T cells recognize virus-infected or cancer cells and destroy them, or serve as helper cells to help B cells produce antibodies. Also included in this group are natural killer (NK) cells, so named for their inherent ability to kill a variety of target cells. In a healthy person, about 25 to 33 percent of white blood cells are lymphocytes.

Four methicillin-resistant Staphylococcus aureus (MRSA) bacteria (purple) engulfed by neutrophils (blue), which are a type of human white blood cell.

Granulocytes, the most numerous of the white cells, rid the body of large pathogenic organisms such as protozoa or helminths and are also key mediators of allergy and other forms of inflammation. These cells contain many cytoplasmic granules or secretory vesicles that contain potent chemicals important for the immune response. They also have multilobed nuclei and are therefore often called polymorphonuclear cells. Based on how their granules take up the dye in the laboratory, granulocytes are divided into three categories: neutrophils, eosinophils, and basophils. The most numerous granulocytes—comprising 50 to 80 percent of all white cells—are neutrophils. They are often one of the first types of cells to arrive at the site of infection, where they engulf and destroy infectious microorganisms through a process called phagocytosis. Eosinophils and basophils, as well as tissue cells called mast cells, usually arrive later. The granules of basophils and closely associated mast cells contain a number of chemicals, including histamine and leukotrienes, that are important in triggering allergic inflammatory responses. Eosinophils destroy parasites and also help modulate inflammatory responses.

Learn about the phagocytosis of the human immune response by viewing a photograph of a leukocyte or white blood cell (globular structure) engulfing bacteria.

White Blood Cell

Monocytes, which make up between 4 and 8 percent of the total number of white blood cells in the blood, travel from the blood to sites of infection, where they further differentiate into macrophages. These cells are scavengers that phagocytose whole or killed microorganisms and are therefore effective in directly killing pathogens and clearing cellular debris from sites of infection. Neutrophils and macrophages are the main phagocytic cells of the body, but macrophages are much larger and longer-lived than neutrophils. Certain macrophages are important as antigen-presenting cells, cells that phagocytose and degrade microbes and present parts of these organisms to T lymphocytes, thereby activating the specific acquired immune response. Your immune system is your body’s first line of defense against invaders like germs. It helps keep you from getting sick and promotes healing when you are unwell or injured. You can boost your immune system by eating nutritious foods, exercising and getting enough sleep.

Your immune system is a large network of organs, white blood cells, proteins and chemicals. All of these parts work together to protect you from germs and other invaders. Your immune system also helps your body heal from infections and injuries.

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But things don’t always go so smoothly. Sometimes your immune system doesn’t work properly. For example, it may be too weak to fight off invaders, or it may respond too strongly.

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Many different conditions can weaken your immune system and make you more susceptible to infections. Conditions at birth are less common than those that develop later in life, such as type 2 diabetes and cancer.

At the other end of the spectrum, your immune system may overreact to invaders (real or perceived). Can launch an attack when there is no invader. Or it can continue to attack after getting rid of an invader. An overactive immune system can lead to problems such as autoimmune diseases or allergic reactions.

The different organs, tissues and cells of your immune system are distributed throughout your body. All are working

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