Immune Cells

A leukocyte is also called a white blood cell (WBC) and represents a type of formed element in blood that aids the immune process and protects the body from infections and foreign invasion. Leukocytes lack hemoglobin and are therefore colorless, or "white," which is why they are also referred to as white blood cells. These cells originate from hematopoietic stem cells found in the red bone marrow that are able to differentiate into many different types of cells, including leukocytes. There are five main types of leukocytes—neutrophils, monocytes, eosinophils, basophils, and lymphocytes—all of which play an important role in the body's immune response. Each neutrophil, eosinophil, and basophil is called a granulocyte, an immune cell that contains granules in the cytoplasm. A neutrophil is the most common type of leukocyte in the body. A neutrophil is a white blood cell that can travel anywhere in the body and is the first to arrive at the site of inflammation or injury. Neutrophils often cause swelling to protect damaged cells. Neutrophils are phagocytic cells because they engulf and digest pathogens and other material. Roughly 40–75% of the leukocytes in blood are neutrophils. The granules in neutrophils stain to a light pink color that can be used to identify them from other immune cells under a microscope. They are normally found in the bloodstream, but during an inflammatory response neutrophils follow chemical signals and quickly migrate to the site of inflammation. This process is referred to as chemotaxis, which involves the movement of a cell in response to a chemical stimulus. There, neutrophils engulf and destroy pathogens, leading to the formation of pus, which is white/yellow in color.

Like a neutrophil, a monocyte is also phagocytic. Recognized as the largest type of leukocyte in the body, a monocyte is a type of leukocyte (white blood cell) that can change as needed into a macrophage targeted to destroy specific foreign substances, such as bacteria and viruses. A macrophage is a monocyte that has migrated from the bloodstream into any tissue in the body. Monocytes circulate in the bloodstream until chemical signals attract them into tissues, at which point the monocytes differentiate into phagocytic cells that patrol tissues in search of pathogens. In addition to engulfing and destroying pathogens in a nonspecific manner, macrophages—a type of phagocytic cell that monocytes differentiate into—also play a role in adaptive immunity. After engulfing a pathogen, macrophages present a portion of that pathogen to special leukocytes called T cells. This activates the specific immune response.

An eosinophil is a leukocyte (white blood cell) from the myeloid lineage that contains granules, releases cytotoxic chemicals to kill large parasites, and plays a role in allergies and asthma. Making up just 0–4% of the body's leukocytes, these cells are not phagocytic. They perform their immune function by releasing toxic chemicals to kill large parasites, such as worms that are too big for a single leukocyte to engulf. Eosinophils play a role in allergies and asthma, similar to basophils, which are immune cells derived from the myeloid lineage that live for a long time in the body's tissues and release chemicals, triggering allergic-mediated responses from the immune system. Symptoms associated with this type of response include a runny nose and itchy eyes.

Also of the myeloid lineage, a basophil is a type of white blood cell that releases histamine and heparin to promote inflammatory reactions such as in allergic reactions, anaphylaxis, and asthma. Basophils make both histamine and serotonin. Basophils are identified by their ability to absorb blue stain. Recognized as the rarest type of leukocyte, a basophil's release of histamine, which is a type of protein molecule also released from mast cells, triggers allergic responses following the invasion of a foreign pathogen into the body. Both histamine and heparin promote inflammation and also play a role in asthma and allergies. Histamine promotes inflammation by dilating blood vessels and thereby increasing the blood flow to injured or infected tissues. Heparin is an anticoagulant that is released by basophils to make it easier for leukocytes to enter the area of the injury.

A lymphocyte is one of a family of several different types of leukocytes (white blood cells), including B cells, T cells, and natural killer (NK) cells. Lymphocytes are found in lymphoid organs, such as lymph nodes. The three types of lymphocytes most important to the immune system are B cells, T cells, and natural killer (NK) cells. Of these lymphocytes, B cells and T cells work together to carry out specific, adaptive immunity. A B cell is a lymphocyte that originates and matures in the red bone marrow and produces antibodies, which bond to pathogens and neutralize them.

When a B cell recognizes an invader, it produces large quantities of antibodies, the large proteins used by the adaptive immune system to target pathogens for destruction. An antibody is a Y-shaped protein that recognizes and binds to a specific antigen, which is a substance on the surface of the pathogen that is recognized by surface antibodies (B cells) or other immune cells to prompt an immune response. An antigen-bound complex is a molecule formed when an antibody binds with an antigen and works as a single entity in combating pathogens. Some antibodies develop naturally as a person contracts an illness. Antibodies are created against the pathogen causing that illness, and thus, the person acquires a natural resistance to that illness in the future.

The body can also develop antibodies to prevent illness through a process called vaccination. A vaccine, a substance that encourages the production of antibodies, is introduced to the body to provide resistance to the pathogen causing the illness. Vaccines are commonly given to babies, young children, and people traveling to areas where specific diseases are prevalent. An infant, for example, would be given vaccines to prevent measles, mumps, rubella, and chicken pox, all highly communicable diseases.

An antibody is also known as an immunoglobulin, a type of protein that can be secreted from or bound to the surface of B cells and recognize antigens. There are five classes of immunoglobulins, whose primary function in the body is to provide an automatic response to the presence of pathogens. The most common immunoglobulin in the body, IgG, accounts for 75% of all immunoglobulins and is found primarily in blood and lymph.

Not all B cells secrete or are bound to immunoglobulin. Some B cells also differentiate to become memory B cells. Memory B cells are long-lived cells that play a key role in adaptive immunity. When memory B cells detect the presence of a pathogen they have encountered before, they quickly replicate to produce large numbers of new cells targeted toward the familiar pathogen.

A T cell is a leukocyte produced by red bone marrow that migrates to the thymus gland, where it matures. T cells play a role in eliciting the adaptive immune response. There are several types of T cells, which include the following:

• Helper T cells (CD4) produce cytokines, which are secreted by the cell to alter the effect on a neighboring cell. For example, certain cytokines, such as interleukin 4 (IL-4), activate B cells to produce antibodies as a mounting immune defense.
• Killer T cells, or cytotoxic T cells (CD8), release cytotoxic granules that kill infected cells.
• Memory T cells are similar to memory B cells in that they are long-lived cells playing a key role in adaptive immunity.

Unlike B cells and T cells, NK cells are lymphocytes that are part of the nonspecific, innate immune response. These cells recognize infected cells and tumors by detecting changes in cell-surface proteins. Once activated, NK cells release cytotoxic granules to kill the infected or diseased cell. The NK cell also has inhibitory membrane proteins that prevent the release of cytotoxic granules when the NK cell identifies other lymphocytes.