Immune Response Health Article

Definition

The action taken by the body to defend itself from pathogens or abnormalities is called the immune response. With the aid of the immune system, the body monitors constant exposure to harmful elements in the external and internal environment and provides a means of defense. Pathogens that are able to cause immune responses included bacteria, viruses, and parasites. The immune system must be able to determine what is a normal part of the body or "self," as opposed to that which is foreign or "non-self." The development of cancers, for example, represents a part of "self" that has been abnormally changed such that it is recognized as foreign to the immune system.

Description

The immune response can be roughly divided into two broad categories, innate (natural) immunity and adaptive (acquired) immunity. Innate immunity is the first line of defense against invasion by pathogens. This response is not directed against any one particular pathogen but is a capable of destroying many different invaders. If the pathogen is able to conquer this initial protection, an adaptive immune response will follow. In this response, lymphocytes arise that can specifically kill the invader and prevent re-infection. These lymphocytes recognize specific antigens on pathogens (substances that are foreign to the host cell and cause the production of antibodies to fight the disease).

Innate (natural) immunity

Innate immunity refers to those parts of the immune system that are normally present and do not given an elevated response upon a second exposure to a pathogen (without immunological memory). This immunity is non-specific and is not directed against one type of pathogen. It is more generalized to allow the recognition of common elements that may be shared among pathogenic microorganisms.

Anatomical or physical barriers provide innate protection. The skin provides a protective barrier and contains substances that are antimicrobial (against bacterial growth) such as lactic acid, ammonia, and uric acid. The bacteria (microflora) that normally inhabit the skin do not cause disease under normal conditions. These organisms also contribute to innate immunity. The competition of the microflora with pathogens for resources and nutrients limits the growth of pathogens. If the skin is broken due to wounds or burns, pathogenic bacteria may enter to cause disease. In the urinary and biliary tracts, the increased flow of secretions provides protection against the establishment of harmful organisms.

Physiologic barriers are also a part of the innate immune system. Stomach acid can kill and inhibit the growth of many microorganisms and degrade potentially harmful proteins. A rise in body temperature can create an environment that is no longer suitable for the growth of some bacteria. Saliva, nasal secretions, tears, and mucus also contain substances that block viruses and help in the destruction of harmful bacteria.

Some cells of the immune system are able to attack and engulf pathogens, molecules, or particles by a process known as phagocytosis. The Russian immunologist Eli Metchnikoff observed that some pathogenic microorganisms were destroyed by phagocytic cells he called macrophages. These phagocytic cells originate in the bone marrow, are called monocytes in the bloodstream, and become macrophages in the tissue. These phagocytic macrophages in the tissue are able to ingest and destroy some pathogens even though they have not previously encountered them. These cells are capable of migration and are found in many sites throughout the body, including the lymph nodes, spleen, liver, lungs, as well as the peritoneal lining that surrounds the organs and the lungs. Macrophages in the bone are called osteoclasts, in the central nervous system they are called microglia, and in the connective tissue they are known as histiocytes. The neutrophils (polymorphonuclear leukocytes or PMNs) are another type of phagocytic cell that is critically important for innate immunity. These cells are found in great numbers and are one of the most important types of white blood cells found in the bloodstream. They are quickly recruited to the site of infection to engulf pathogens. Both neutrophils and macrophages contain enzymes that break down the engulfed material.

Natural killer (NK) cells are a type of lymphocyte in the blood that can detect and destroy cells infected by certain viruses. Viruses attack host cells and use them to facilitate viral replication and production of more viruses. Infected host cells must be rapidly destroyed to prevent this replication and spread of disease. It has been observed that natural killer cells play an especially important role in the early defense against herpes viruses. They also are involved in the killing of some tumors. Natural killer cells may kill by activating a process called apoptosis, the programmed cell death that is present in all cells and is responsible for their self-destruction.

The plasma contains a group of proteins called complements that act in a coordinated manner to attack pathogens. When some pathogens bind with a complement protein called C3b, a series of reactions in the alternative complement pathway occur. The surface of the pathogen is changed so that phagocytic cells can ingest them, a process called opsinization.

If the pathogen is able to effectively cross the barriers of innate immunity, an early induced, non-adaptive response will occur. This response serves to stop pathogens or slow them down until the body can initiate an adaptive immune response. Additional phagocytic cells and molecules are summoned to the site of infection by cytokines, a group of proteins that affect the actions of other cells. Some cytokines can cause an increase in the number of neutrophils in the circulation and fever, an elevation in body temperature. As most pathogenic bacteria have optimal growth at lower temperatures, this temperature rise helps to inhibit their growth. The fever also enhances the adaptive immune responses that follow. Local effects from injury or infection give rise to inflammation as white blood cells, fluid, and plasma proteins gather at the site. This is evident clinically at the site by redness, pain, heat, and swelling. The blood vessels in the site of injury or infection increase in diameter and allow more blood to flow into the area at a slower rate. Immune cells arrive quickly to the site and move into the tissue from the bloodstream. Small proteins called chemokines assist in this process and enhance the migration and activation of cells. Other special proteins called interferons are produced by virally infected cells and may stop the virus from multiplying within other cells, preventing the spread of infection.