Diseases & Conditions


Innate Immunity

Innate (natural) immunity is so named because it is present at birth and does not have to be learned through exposure to an invader. It thus provides an immediate response to foreign cells. However, its components treat all foreign substances in much the same way. They recognize only a limited number of identifying substances (antigens) on foreign cells, although these antigens are present on many different cells. Innate immunity has no memory of the encounters and does not provide any lasting protection against future infection.

The white blood cells involved in innate immunity are Monocytes (which develop into macrophages) Neutrophils Eosinophils Basophils Natural killer cells

Each type has a different function. The complement system and cytokines also participate in innate immunity.


Macrophages develop from a type of white blood cell called monocytes after monocytes move from the bloodstream to the tissues. Monocytes move to the tissues when infection occurs. There, over a period of about 8 hours, monocytes enlarge greatly and produce granules within themselves, becoming macrophages. The granules are filled with enzymes and other substances that help kill and digest bacteria and other foreign cells. Macrophages stay in the tissues. They ingest bacteria, foreign cells, and damaged and dead cells. (The process of a cell ingesting a microorganism, another cell, or cell fragments is called phagocytosis, and cells that ingest are called phagocytes.)

Macrophages secrete substances that attract other white blood cells to the site of the infection. They also help T cells recognize invaders and thus participate in acquired immunity.



Neutrophils, the most common type of white blood cell in the bloodstream, are among the first immune cells to defend against infection. They ingest bacteria and other foreign cells. Neutrophils contain granules that release enzymes to help kill and digest these cells.

Neutrophils circulate in the bloodstream and must be signaled to leave the bloodstream and enter tissues. The signal often comes from the bacteria themselves, from complement proteins, or from damaged tissue, all of which produce substances that attract neutrophils to a trouble spot. (The process of attracting cells is called chemotaxis.)

Neutrophils also release substances the produce fibers in the surrounding tissue. These fibers may trap bacteria, thus keeping them from spreading and making them easier to destroy.



Eosinophils can ingest bacteria but also target foreign cells that are too large to ingest. Eosinophils contain granules that release enzymes and other toxic substances when foreign cells are encountered. These substances make holes in the target cell's membranes.

Eosinophils circulate in the bloodstream. However, they are less active against bacteria than are neutrophils and macrophages. Their main function may be to attach to and thus help immobilize and kill parasites.

Eosinophils help destroy cancer cells. They also produce chemicals involved in inflammation and allergic reactions (such as asthma—see Allergic Reactions: Symptoms ). People with allergies, parasitic infections, or asthma often have more eosinophils in the bloodstream than people without these disorders.


Basophils do not ingest foreign cells. They contain granules filled with histamine, a substance involved in allergic reactions. When basophils encounter allergens (antigens that cause allergic reactions), they release histamine. Histamine increases blood flow to damaged tissues. Basophils also produce substances that attract neutrophils and eosinophils to a trouble spot.

Natural Killer Cells

Natural killer cells are called “natural” killers because they are ready to kill as soon as they are formed. Natural killer cells attach to foreign cells and release enzymes and other substances that damage the outer membranes of the foreign cells. Natural killer cells kill certain microorganisms, cancer cells, and cells infected by viruses. Thus, natural killer cells are important in the initial defense against viral infections.

Also, natural killer cells produce cytokines that regulate some of the functions of T cells, B cells, and macrophages.

Dendritic Cells

Dendritic cells reside in the skin, lymph nodes, and tissues throughout the body. Most dendritic cells ingest and break antigens into fragments (called antigen processing), enabling helper T cells to recognize the antigen. Dendritic cells present antigen fragments to T cells in the lymph nodes.

Another type of dendritic cell, the follicular dendritic cell, presents unprocessed (intact) antigen that has been linked with antibody (antibody-antigen complex) to B cells.

After T and B cells are presented with the antigen, they become activated.

Complement System

The complement system consists of more than 30 proteins that act in a sequence: One protein activates another and so on. This sequence is called the complement cascade. Complement proteins have many functions in acquired immunity as well as innate: Killing bacteria directly Helping destroy bacteria by attaching to them and thus making the bacteria easier for neutrophils and macrophages to identify and ingest Attracting macrophages and neutrophils to a trouble spot Causing bacteria to clump together Neutralizing viruses Helping immune cells remember specific invaders Promoting antibody formation Enhancing the effectiveness of antibodies Helping the body eliminate immune complexes, which consist of an antibody attached to a foreign substance (antigen), and dead cells


Cytokines are the messengers of the immune system. White blood cells and certain other cells of the immune system produce cytokines when an antigen is detected.

There are many different cytokines, which affect different parts of the immune system. Some stimulate activity. They stimulate certain white blood cells to become more effective killers and to attract other white blood cells to a trouble spot. Other cytokines inhibit activity, helping end an immune response. Some cytokines, called interferons, interfere with the reproduction (replication) of viruses. Cytokines also participate in specific immunity.

Last full review/revision September 2008 by Peter J. Delves, PhD

Source: The Merck Manual Home Edition