White blood cell

White blood cells are a component of blood. As part of the body's immune system, their function is to protect the body from disease and infection if pathogens get in the bloodstream. White blood cells come in several varieties, each with a different role to play in neutralizing harmful foreign substances. White blood cells, like all blood cells, are produced in the bone marrow.

They are also known as leukocytes. There are many types of leukocytes, and these types are produced from immature cells in the lymphatic system in response to the body's needs. For example, infection with bacteria will cause more macrophages to be produced and fewer NK cells, in response to "alarm signals" called chemokines. Macrophages then exit the bloodstream, migrate towards the site of infection, and attempt to neutralize the invading pathogens. Others begin producing antibodies or recruit other systems to help.

The immune system of all vertebrates has memory, which in this context means that it keeps a record of invaders it has previously encountered. Immune memory is not completely understood, but involves the cells that contributed most to fighting off the infection being stored away in the bone marrow. This system is why you will not usually get chickenpox if you have already had it, and is also why vaccines work (the vaccine contains a tiny, harmless bit of a pathogen such as polio, so the immune system will "remember" it and then be ready if it should ever encounter the real thing).

Outline of Types

There are many different kinds of immune cells. The most common types, and their functions, are outlined below:

1. Granulocytes, innate immune cells and antigen presenting cells.

A. Macrophages

• Phagocytose pathogens and lyse foreign cells
• As antigen presenting cells, primarily present antigens for T-cell activation
• Elevated count generally indicates infection
• Release inflammatory cytokines:

• IL-1
• IL-6
• TNF-α

B. Dendritic cells

• Immature dendritic cells, “Langherhan’s cells”, reside in interstitial space
• Langherhan's cells are constantly sampling interstitial fluid via macropinocytosis (cell "drinking")
• When activated, become migratory “veiled cells” and migrate to lymph nodes to function as APCs.
• Stimulate clonal expansion of CD4 cells ("helper" T cells)

C. Eosinophil

• An "inflammatory granulocyte"
• When activated, secrete cytotoxic proteins and peroxidases to kill pathogens
• Also secrete antiviral RNAases
• Elevated count generally indicates parasitic infection but also commonly indicate Hodgkin’s disease, and rheumatoid arthritis
• Possess secretory granules containing:

• Major Basic Protein
• Eosonophil Cationic Protein
• Inflammatory Cytokines

D. Neutrophil

• Most numerous innate immune cell
• Phagocytose and lyse pathogenic particles
• Depressed count is associated with many immune deficiencies
• Possess secretory granules containing:

• Lactoferrin
• Lysozymes
• Collagenase
• Myeloperoxidase

E. Basophil

• Secrete inflammatory cytokines
• Release IL-4 to induce T_H2 cell activation

F. Mast cells

• Implicated in allergies

• Characterized by surface IgE-FcεR1 complex

• When activated:

• Release granules of histamine

2. Lymphocytes, all derived from the common lymphoid progenitor, comprise the cells responsible for adaptive immunity as well as some types innate immune cells.

A. NK cells

• “Natural Killer” cells
• Innate immune cells that identify and kill cells expressing abnormal surface markers due to viral infection or cancer.
• Elevated count generally indicates viral infection or cancer

B. T Cells

• Adaptive immune cells
• Several known types, some (e.g. intraepithelial γδ T-cells) may primarily function in innate immunity
• Antigenic specificity determined by complimentarity determining region of T cell receptor
• Two Major Sub-Populations, distinguished by expression of either CD4 or CD8 co-receptor:

1. CD4+ cells, comprise several different cell types:

a. T_H1

• “Inflammatory” T cells
• Development favored by IL-2 and IFN-γ
• Secrete Type 1 Cytokines:

• IL-2
• TNF-β
• INF-γ

b. T_H2

• “Helper” T cells
• When stimulated, present antigen to and activate B cells
• Development favored by MCP-1 and IL-4
• Secrete Type 2 Cytokines:

• IL-4
• IL-5
• IL-10

C. T_H17

• Mediate Inflammation
• Stimulate epithelial cells to produce anti-microbial proteins
• Stimulate neutrophils

D. T_reg

• Suppress lymphocyte activate
• Express CD25 and Foxp3 surface markers

2. CD8+

• “Killer” T cells
• T cell receptor on surface interacts with class I MHC molecule on target cells, induces cell death in target cell if target cell is presenting complementary antigen
• Secrete inflammatory cytokines:

• INF-γ
• TNF-α
• TNF-β

See Also

• Red blood cell
• Platelets