The immune systems of all animals include innate immunity, a set of defenses that are active immediately upon infection and are the same whether or not the pathogen has been encountered previously.

Natural killer cells recognize cancer cells and virus-infected cells and release chemicals that cause the death of these cells.

Macrophages are large phagocytes that wander through the interstitial fluid, “eating” any bacteria and viruses they encounter.

Tissue damage triggers the inflammatory response, which can disinfect tissues and limit further infection.

The main function of the inflammatory response is to disinfect and clean injured tissues.

➌ The neutrophils that migrate into the area engulf bacteria and the remains of any body cells killed by them or by the physical injury. Many of the neutrophils die in the process (or simply come to the end of their short life span), and their remains are also engulfed and digested.

Infections and vaccinations trigger adaptive immunity.

Antigens may also be substances released into the extracellular fluid, such as toxins secreted by bacteria.

An antibody is an immune protein found in blood plasma that attaches to one particular kind of antigen and helps counter its effects.

For example, a fetus obtains its mother’s antibodies through the placenta; after birth, breast milk supplies the baby with antibodies. The effects of a poisonous snakebite may be counteracted by injecting the victim with antivenom, which consists of antibodies extracted from animals previously immunized against the venom. Passive immunity is temporary because the recipient’s immune system is not stimulated by antigens. Immunity lasts only as long as the antibodies do; after a few weeks or months, these proteins break down and are recycled by the body.

Lymphocytes are the white blood cells responsible for adaptive immunity.

Lymphatic vessels collect fluid from body tissues and return it as lymph to the blood. Lymph organs are packed with white blood cells that fight infections.

The lymphatic vessels carry a fluid called lymph, which is similar to the interstitial fluid that surrounds body cells but contains less oxygen and fewer nutrients.

The lymphatic system is closely associated with the circulatory system. Most infectious agents wind up in the circulatory system. From there they are carried into the lymphatic system, which can usually filter them out. The filtered fluid can then be recycled into the circulatory system. The lymphatic system thus has two main functions: to return tissue fluid to the circulatory system and to fight infection.

Millions of kinds of B cells and T cells, each with different membrane receptors, wait in the lymphatic system, where they may respond to invaders.

Some immature lymphocytes continue developing in the bone marrow to become specialized as B lymphocytes, or B cells.

The proteins are antigen receptors, capable of binding one specific type of antigen.

One of the two adaptive responses, produced by B cells, is the humoral immune response, which defends primarily against bacteria and viruses present in body fluids. In this response, B cells secrete free-floating antibodies into the blood and lymph.

Other immature lymphocytes migrate to the thymus, a gland above the heart, to become specialized as T lymphocytes, or T cells. By mounting a dual defense, B and T cells defend against infections in body fluids and cells.

The second type of adaptive immunity, produced by T cells, is the cell-mediated immune response, which defends against infections inside body cells. This defensive system results from the action of defensive T cells, in contrast to the action of free-floating defensive antibody proteins produced by B cells of the humoral response.

The site on the antigen that antibodies and antigen receptors bind to is the epitope.

A small surface-exposed region of an antigen is called an epitope.

Antigen receptors on B cells, as well as antibodies, recognize and bind to the epitope. The specific region on an antigen receptor or antibody that recognizes an epitope is the antigen-binding site.

Both the humoral and cell-mediated immune responses are initiated when lymphocytes recognize antigens. B cells bind antigens directly, whereas T cells require an additional step for recognition.

Antigens usually do not belong to the host animal. Most antigens are proteins or large polysaccharides that protrude from the surfaces of viruses or foreign cells.

When an antigen enters the body, it activates only a small subset of lymphocytes that have receptors specific for the antigen. The selected cells multiply into clones of short-lived effector cells specialized for defending against that antigen and into memory cells, which confer long-term immunity.

The humoral and cell-mediated immune responses both defend against a wide variety of antigens through a process known as clonal selection.

Some of these cells, called effector cells, act immediately to combat infection, while others known as memory cells lie in wait, ready to help activate the immune system upon subsequent exposure to the antigen.

This antigen-driven cloning of lymphocytes clonal selection is a vital step in the adaptive immune response against infection.

The primary immune response occurs the first time a particular antigen enters the body and selectively activates lymphocytes.

When memory cells produced during the primary response are activated by a second exposure to the same antigen which may occur soon or long after the primary immune response they initiate the secondary immune response.

The first exposure to an antigen results in the primary response. In a second exposure, memory cells initiate a faster, stronger, and more prolonged response.

When most people in a population are vaccinated, a disease cannot spread.

This community protection, referred to as herd immunity, is the rationale behind state-mandated vaccinations for children in public schools.

An antibody has antigen-binding sites that bind to specific antigens. When bound to antigens on the surface of foreign cells, antibodies assist innate responses in eliminating the invader.

An antibody has two related functions in the humoral immune response: first, to recognize and bind to a specific antigen and, second, to assist in eliminating that antigen. Both of these functions are facilitated by the antibody’s molecular structure.

The tip of each arm of the Y forms an antigen-binding site, a region of the molecule responsible for the antibody’s recognition-and-binding function.

All antibody mechanisms involve two parts: a specific recognition-and-attach phase followed by a nonspecific destruction phase. Thus, antibodies of the adaptive humoral immune response, which identify and bind to invaders, must work with components of innate immunity.

Several vaccines have been developed to promote active immunity before individuals come into contact with cancer- causing strains of HPV.

Active adaptive immunity to a specific pathogen can be gained through a natural infection or through vaccination.

The first two vaccines approved in the United States were Gardasil and Cervarix. Both of these vaccines are made with HPV antigens.

To determine the effectiveness of these vaccines, scientists have been conducting controlled studies. In these studies, participants are randomly assigned to one of two groups; those in the experimental group receive an injection of a vaccine, while those in the control group are injected with a placebo.

To measure long-lasting antibody production against HPV, scientists designed and carried out two long-term studies.

Scientists hypothesize that higher levels of anti-HPV antibodies provide greater protection from HPVrelated cancers than lower levels of antibodies, but they don’t yet have strong evidence to support this hypothesis.

The role of helper T cells is so central to immunity that without functional helper T cells, there is virtually no immune response.

First, there must be a foreign molecule that can bind specifically to the antigen receptor of the T cell.

Second, this antigen must be displayed on the surface of an antigen-presenting cell. Macrophages and B cells are two types of antigen-presenting cells.

➊ First, they stimulate clonal selection of the helper T cell, producing both memory cells and additional effector helper T cells.

➋Second, the signaling molecules help activate B cells, thus stimulating the humoral immune response.

➌Third, the signals stimulate the activity of cytotoxic T cells of the cell-mediated immune response, our next topic.

➊A cytotoxic T cell with a matching receptor binds to an infected body cell. The T cell then synthesizes several toxic proteins that act on the bound cell, including one called perforin.

➋ Perforin is discharged from the T cell and attaches to the infected cell’s plasma membrane, forming pores in it. T cell enzymes, which enter the infected cell by endocytosis, break down proteins.

➌ The breakdown of proteins kills the infected cell. The death of the infected cell deprives the pathogen of a place to multiply and also exposes the contents of the infected cell to circulating antibodies, which mark the released antigens for disposal.

➍ The cytotoxic T cell may move on to destroy other cells infected with the same pathogen.

Cytotoxic T cells play a role in protecting the body against the spread of some cancers.

AIDS (acquired immunodeficiency syndrome) results from infection by HIV, the human immunodeficiency virus.

Immune system impairment makes AIDS patients susceptible to cancers and opportunistic infections, which are infections that can normally be fought off by a person with a healthy immune system.

Drugs, vaccines, and education are areas of focus for the
prevention of HIV infection. Drug development has led to a drastic reduction in transmission rates of HIV from mother to child.

Currently, the most effective form of prevention is education. People learn to avoid direct contact with blood (especially through shared intravenous drug needles). Infected mothers are taught precautions to keep from transmitting the disease to their babies. Sexually active individuals are taught that HIV is transmitted during sexual activities and how condoms can be used to prevent the spread of the virus. Safe sex alone could save millions of lives.

As HIV reproduces, mutations occur, some of which can generate new strains of the virus. In fact, the virus mutates at a very high rate during replication because reverse transcriptase does not have an editing function to correct mistakes.

If a virus strain becomes resistant to one drug, it may be defeated by another drug, so patients are typically given multidrug regimens. Unfortunately, some HIV strains have already evolved resistance to multidrug regimens.

. The resistant virus may mutate during this waiting period, causing it to become susceptible to the drugs again. The battle continues, with medical science on one side and the constantly evolving HIV on the other.

The ability of lymphocytes to recognize the body’s own molecules to distinguish self from nonself enables our adaptive immune response to battle foreign invaders without harming healthy cells.

The immune system’s ability to recognize foreign antigens does not always work in our favor. When a person receives an organ transplant or tissue graft, the person’s T cells recognize the MHC markers on the donor’s cells as foreign.

In addition, doctors use drugs to suppress the immune response against the transplant. Unfortunately, these drugs may also reduce the ability to fight infections and cancer.

In autoimmune diseases, the immune system targets self molecules. In immunodeficiency disorders, immune components are lacking and frequent infections occur.

Lupus is characterized by skin rashes, fever, arthritis, and kidney malfunction.

Symptoms of rheumatoid arthritis include damage to and painful inflammation of the cartilage and bone of joints.

Immunodeficiency may be acquired later in life, as in the case of AIDS. Another example of an acquired immunodeficiency is Hodgkin’s disease, a type of cancer that damages the lymphatic system and can depress the immune system.

Allergies are hypersensitive (exaggerated) responses to otherwise harmless antigens in the environment.

Antigens that cause allergies are called allergens. Common allergens include protein molecules on pollen grains and on the feces of tiny mites that live in house dust.

➍ The allergen enters the body and binds to the antibodies attached to mast cells.

➎ This causes the mast cells to release histamine, which causes blood vessels to dilate and leak fluid, leading to nasal irritation, itchy skin, and tears.

➊ After an allergen enters the bloodstream, it binds to effector B cells with receptors specific to the allergen.

➋ The B cells then proliferate through clonal selection and secrete large amounts of antibodies to this allergen.

➌ Some of these antibodies attach to the surfaces of mast cells that produce histamine and other chemicals, which trigger the inflammatory response.

Antihistamines are drugs that interfere with histamine’s action and give temporary relief from an allergy.