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Overview of White Blood Cells and Immunity


Updated June 25, 2014

Our immune system, or our immunity, is a mechanism that protects our bodies from harmful substances, foreign microorganisms, and even cancer. While originally our immune system was studied in terms of its role in preventing infections, our understanding has evolved and led to a broader view of blood and marrow stem cell transplantation, blood transfusion, cancer, and genetics. Our white blood cells (WBCs) are a part of our immune system and play a prolific role in blood and marrow cancers. In fact, the word leukemia means “white blood,” as it is related to an overproduction of white blood cells.

The immune system has four main functions:

  • Protection against foreign microorganisms
  • Homeostasis through the removal of worn out or dead cells
  • Surveillance and removal of mutant cells
  • Regulation through increasing and suppressing immune response

The immune system is capable of carrying out these functions primarily through its ability to tell the difference between self (part of the body) and non-self (invading organisms like bacteria, fungus, and viruses, or toxins) cells. It determines this by the antigens, or proteins, on the cell surface. Cells with antigens that the immune system recognizes as the self are left alone, while a non-self antigen cell will deploy our immunity, which will respond by recruiting, activating and mobilizing the appropriate white blood cells to the location of the threat.

The two basic types of immunity are innate and acquired immunity. Some of our white blood cells play a role in innate immunity, others in acquired immunity, while some are involved in both.

Innate Immunity

Innate immunity is the first-line, non-specific response to any breach of our bodies. We are born with innate immunity. Innate immunity is carried out by four mechanisms: mechanical barriers, chemical barriers, fever, and phagocytosis or inflammation.

Mechanical barriers include the skin and membranes that line our mouths, nose, airways, urinary tracts, and gastrointestinal organs. When these are intact, they provide a physical barrier against the entry of toxins and harmful organisms.

Chemical barriers include sweat, tears, saliva, stomach acids, mucus, and other fluids secreted by the body. These prevent foreign invaders by making the environment inhospitable to them. For example, chemical barriers may make a tissue too acidic or sticky and thus deter the organism or toxin from attaching and inhabiting the body.

Fever helps the body defend against bacteria and viruses that are sensitive to extremes in temperature. These invaders excrete substances that trigger the body into increasing its temperature. These types of organisms cannot tolerate the elevated temperatures for an extended period of time.

Inflammation occurs when the mechanical and chemical barriers to foreign invaders have failed. The response is very fast, but also very short-lived. This is a non-specific response, that is, your body does not have to have been exposed to this invader before to respond to it. Inflammation is initiated and controlled by phagocytic WBCs, such as polymorphonuclear leukocytes or “granulocytes”-- neutrophils, monocytes, macrophages, eosinophils, basophils, and natural killer cells.

Neutrophils attack organisms such as bacteria and fungi, and may leave the blood for the tissues during an inflammatory response. Basophils initiate an inflammatory response to environmental antigens, whereas eosinophils defend the body against parasites. Natural killer cells use potent chemicals to kill infected cells on contact.

Macrophages act as scavenger cells in the tissues, while monocytes perform their role in the bloodstream. Once a macrophage digests a foreign invader, it presents details about that organism to T-lymphocytes and helps to recruit the acquired immune system.

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