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Did You Know?
Normally, sperms have a coating of glycoprotein molecules on their surface that averts an attack by the white blood cells in the female reproductive tract. The absence of such a coating would result in sperms being killed-off before reaching the ovum.

White Blood Cells
Leukocytes, or white blood cells (WBCs), include five basic types of cells - neutrophils, eosinophils, basophils, lymphocytes and monocytes. These are broadly grouped into agranulocytes and granulocytes, based on the absence or presence of specific staining granules. Agranulocytes include lymphocytes and monocytes, while granulocytes include neutrophils, eosinophils and basophils. Lymphocytes are further divided into B cells, T cells and natural killer (NK) cells. Monocytes give rise to macrophages, whose main function is to ingest and destroy foreign particles and organisms.

White Blood Cells

Under the Microscope...
All cells are colorless, so to visualize them under the microscope, they are stained with different dyes. Depending upon the chemical nature of the dye, certain components of the cells acquire the stain and stand out. Neutrophils appear pink under the microscope because they stain with neutral dyes. Eosinophils appear red owing to their tendency of taking up the acidic stain eosin. Basophils appear blue because they take up methylene blue, which is a basic dye.

Shape-wise too, these cells differ from one another. Neutrophils have multi-lobed nuclei, eosinophils have bi-lobed nuclei and basophils may have 2-4 nuclei.When seen under the microscope, agranulocytes do not appear to have lobed nuclei (a nucleus is a part of the cell that houses its genetic material).

White blood cells derive their name from the fact that all immune cells separate in a single layer upon centrifugation of blood. This layer is white in color, hence the name leukocytes.

Neutrophils

Neutrophils constitute around 50-70% of all leukocytes. They are also known as polymorphonuclear (PMN) cells and are short-lived (4-5 days). They mostly fight infections caused by bacteria and fungi, and are the first cells of the immune system that arrive at a site where a new pathogen (disease-causing microbe) or new antigen (immune system-activating entity) present themselves. When an infection occurs, neutrophils traveling in the blood vessels close to the site of infection are attracted to the site by chemicals released by the microbe as well as by other immune cells. After reaching the site, neutrophils surround and ingest the microbe. The granules present in neutrophils contain several chemicals, mostly enzymes, for destroying ingested microbes.

Increased in:
  • Bacterial or fungal infections
  • Therapy with steroids
  • Inflammation
  • Leukemias (acute and chronic)
  • Stress
  • Polycythemia vera
  • Myocardial infarction
  • Non-pathological conditions such as after exercise and in pregnancy.
  • Leukocyte Adhesion Deficiency
  • Certain types of anemias
Decreased in:
  • Chediak-Higashi Syndrome
  • People of African ethnicity have lower neutrophil counts.
  • Certain viral infections
  • Felty's Syndrome
  • Myelosuppression
  • Kostmann's Syndrome
  • Cyclical Neutropenia
  • Chronic granulomatous disease
  • Hypersplenism
  • Myeloid leukemias
  • Autoimmune neutropenia
  • Shwachman-Bodian-Diamond Syndrome
  • Pancytopenia

Eosinophils

Eosinophils are immune cells mostly dealing with parasitic infestations in the body. They are active against nematodes and other parasites as well as against protozoa. Since parasitic infestations provoke strong allergic reactions in the body, they are associated with high Immunoglobulin G (IgG) numbers.

Increased in:
  • Parasitic infestations such as Ascariasis, nematode infestation, roundworm infestation
  • All kinds of allergies like hay fever, bronchial asthma, urticaria, eczema, allergic aspergillosis and allergic drug reactions.
  • Churg-Strauss syndrome
  • Impairment of the adrenal gland
  • Sarcoidosis
  • Malignancies such as Hodgkin's disease
  • Certain kinds of leukemias
  • Hypereosinophilic syndrome
  • Eosinophilic Gastrointestinal Disorders
Decreased in
  • Acute infections
  • High levels of stress
  • Cushing's disease

Basophils

Basophils are the least abundant cells of the immune system. They have large granules containing inflammation-causing molecules like histamine and leukotriene, and also contain the anticoagulant heparin. They are one of the cells that cause inflammation when the body is attacked by helminthic parasites. Their granules stain bluish-purple with basic dyes such as Lewandowsky's stain. Basophils, when stimulated by attachment with the antibody IgE, release the contents of their granules, which help to mount an inflammatory reaction. They are also believed to play a role in controlling the activity of T cells.

Increased in
  • Certain types of leukemia
  • Surgical removal of the spleen
  • Viral infections
  • Certain parasitic infestations
  • Myxedema
  • Inflammation
Decreased in
  • Urticaria
  • Hyperthyroidism
  • High levels of stress
  • Chemotherapy for malignancies
  • Radiation therapy for malignancies

Monocytes

These cells serve as the precursors of macrophages (immune cells that ingest and destroy microbes and infected cells). They have a short life span in the blood. In tissues, however, they may persist for years. They have variably-shaped nuclei and contain few granules.

Increased in
  • Chronic infections caused by bacteria (TB, infections caused by Streptococcus)
  • Certain kinds of chronic leukemia
  • Myelodysplasia
  • Chronically low neutrophil count
  • Inflammatory Bowel Disease
Decreased in
  • Hairy Cell Leukemia
  • Bone marrow impairment

B Cells

B cells perform the task of producing antibodies. Antibodies are flexible molecules that can recognize and attach to molecules (known as antigens) on the surface of microbes . The five types of antibodies are IgA, IgD, IgE, IgG and IgM. After an antibody has attached to an antigen molecule, the microbe is ready for destruction. The bone marrow is the site of maturation of B cells. B cells that have been exposed to a microbe produce two kinds of cells, memory cells and plasma cells. Memory cells persist for an individual's lifetime, and are the ones that give a person lifelong immunity to a disease. Plasma cells are, by contrast, short-lived and have the job of secreting huge quantities of antibodies for the immediate tackling of an infection. B cells often act in tandem with T cells to tackle infections.

Increased in:
  • Chronic Lymphocytic Leukemia
  • Multiple
    Myeloma
  • Macroglobulinemia of Waldenstrom
Decreased in:
  • X-linked Hypogammaglobulinemia
  • Congenital Rubella
  • Severe Malnutrition
  • Common Variable Immunodeficiency
  • Immunoglobulin deficiencies (congenital and acquired)
  • Myelomas and Lymphomas
  • Severe Combined Immunodeficiency Syndrome (SCID)

T Cells

T cells derive their name from the fact that they mature in the thymus after leaving the bone marrow. After maturation, T cells leave the thymus and move to lymphoid organs like the lymph nodes and tonsils. They also move to the spleen. They have a different mechanism for recognizing microbes as compared to B cells. They recognize peptide fragments instead of the shape of antigen molecules. There are various subtypes of T cells such as cytotoxic T cells, Helper T cells, Antibody-Dependent Cytotoxic Cells (ADCCs) and Lymphokine-activated killer (LAK) cells.

Cytotoxic T cells
These cells bear molecules known as CD8 on their surface. They destroy cells that have turned cancerous or that have been infected by viruses. They recognize such cells by small molecules expressed on them, that are not expressed on normal ones. Assisting them in this task are helper T cells and other cells of the immune system.

Helper T cells
These cells do not have the ability to destroy infected cells on their own. As the name suggests, they have the function of assisting other immune cells in recognizing the cells to be eliminated. They have molecules known as CD4 on their surface.They release cytokines to induce macrophages that harbor microbes within themselves to self-destruct. Helper T cells are of two types, T helper 1 cells and T helper 2 cells. The T helper 1 cells activate cytotoxic T cells, macrophages and Natural Killer cells. The T helper 2 cells assist B cells.

Antibody-Dependent Cytotoxic Cells (ADCC)
These cells do not have any surface markers. They have receptors that help them identify cells to which immunoglobulins are attached.

Lymphokine-activated killer (LAK) cells
These cells do not occur naturally but are the product of treating lymphocytes with Interleukin-2, so that they become more potent. They have been used in the treatment of certain cancers.

Increased in
  • Leukemias (both acute and chronic)
  • Multiple Myeloma
  • Tuberculosis
  • Infectious Mononucleosis
  • Syphilis
  • Toxoplasmosis
Decreased in
  • DiGeorge Anomaly
  • Signal Transduction Defect
  • Measles
  • Administration of Tacrolimus, corticosteroids
  • Acquired Immunodeficiency Syndrome (AIDS)
  • Severe Combined Immunodeficiency Syndrome (SCID)

Natural Killer (NK) Cells

These cells are part of the innate immune system. They recognize virus-infected cells and cancerous cells by the absence of class 1 MHC molecules (normal cells are class1 MHC positive, meaning that they have this molecule on their surface; tumor cells and virus-infected cells often do not).

How do Microbes Avoid the Immune System

Microbes do not simply allow themselves to be eliminated by our immune systems; if that were so, nobody in the world would suffer from any infectious disease. Instead, microbes continue to evolve in ways that would allow them to better adapt to and to evade their hosts' immune systems. Some of the novel ways they use to accomplish this are:
  • Secreting substances toxic to the host cell
  • Secreting molecules that are very similar to important host molecules; this has the effect of fooling the immune system into considering such molecules as its own
  • Avoiding ingestion by certain immune cells
  • Constantly changing the proteins expressed on their surface to avoid being recognized by immune cells
  • Avoiding presentation of structures (known as "antigens") on their surfaces, again to avoid being recognized by immune cells
  • Suppressing important classes of molecules of the immune system, such as complement, cytokines, interferons and chemokines
  • Altering pathways of the infected host cell, to prevent it from dying (as you know, our immune system kills off infected cells to prevent the infection from spreading; microbes are able to survive by blocking this cell-death)
  • Blocking of activation of genes that play important roles in immune activity
  • Blocking antimicrobial substances produced by immune cells

Disclaimer: This Buzzle article is for informative purposes only, and should not be used as a replacement for expert medical advice.