Chapter 18 - Adaptive Specific Host Defenses

Adaptive Immunity

immunity or resistance to a specific pathogen; slower to respond, has memory component

Primary response

Initial response to a specific antigen. During a primary response, T and B lymphocytes are activated and specific antibodies and memory cells to the antigen produced.

Secondary response

The adaptive immune response provoked by a second exposure to an antigen. It differs from the primary response by starting sooner and building more quickly due to memory

Humoral immunity

specific immunity produced by B cells that produce antibodies that circulate in body fluids

Antibody

A protein that acts against a specific antigen

Antigen

A protein that, when introduced in the blood, triggers the production of an antibody

B cells

Cells manufactured in the bone marrow that create antibodies for isolating and destroying invading bacteria and viruses.

Cellular immunity

immune response that relies on T cells to destroy infected body cells

T cells

Cells that mature in the thymus that produce substances that attack infected cells in the body.

T cell receptor (TCR)

Molecule on the surface of a T cell that can bind to a specific antigen fragment in combination with an MHC molecule.

Cytokine

any of a number of substances, such as interferon, interleukin, and growth factors, that are secreted by certain cells of the immune system and have an effect on other cells.

Interleukins (ILs)

immune system cytokines that signal among leukocytes

Chemokines

A chemical secreted by blood vessel endothelium and macrophages during an immune response to attract phagocytes to an area

Interferons (IFNs)

Cytokines that interfere with viral infections of host cells

Tumor necrosis factor alpha

Secreted by macrophages, mast cells and others in response to stimulation of TLRs. Has pro inflammatory effects, particularly on the vascular endothelium and macrophages. Is involved in inflammation of autoimmune diseases

Hematopoietic cytokines

influence differentiation of blood stem cells

Epitope

the part of an antigen molecule to which an antibody attaches itself

Antigenic determinant

the specific area of an antigen that binds with an antibody combining site and determines the specificity of the antigen-antibody reaction; also called an epitope

Hapten

small molecule that has to bind to a larger molecule to form an antigen

Immunoglobulins

Secreted antibodies; bind with specific antigens in the antigen-antibody response

Valence

the number of antigen-binding sites on an antibody

Structure of immunoglobulins

Y shaped with four polypeptide chains. Two chains are small and called light chains (L) and two are large and called heavy chains (H)

Variable (v) region of antibody

part of antibody that combines with the antigen

Constant (Fc) region

the stem, which is identical for a particular Ig class

IgG

Monomer

80% of serum antibodies

In the blood, lymph, and intestine

Cross the placenta; trigger complement; enhance phagocytosis; neutralize toxins and viruses; protect fetus

IgM

Pentamer made of five monomers held with a J chain

6% of serum antibodies

Remain in blood vessels

Cause clumping of cells and viruses

First response to an infection; short-lived

IgA

Monomer in serum; dimer in secretions

13% of serum antibodies

Common in mucous membranes, saliva, tears, and breast milk

Prevent microbial attachment to mucous membranes

IgD

Monomer

0.02% of serum antibodies

Structure similar to IgG

In blood, in lymph, and on B cells

No well-defined function; assists in the immune response on B cells

IgE

Monomer

0.002% of serum antibodies

On mast cells, on basophils, and in blood

Cause the release of histamines when bound to antigen; lysis of parasitic worms

Major Histocompatibility Complex (MHC)

Group of genes that code for proteins found on the surfaces of cells that help the immune system recognize foreign substances.

MHC I

Found on all nucleated cells of body

Identify self

MHC II

Expressed on the surface of APCs (macrophages, dendritic cells, B cells) and functions by presenting antigen that is foreign in the body.

Antigen-presenting cells (APCs)

Cells such as B cells, macrophages, and dendritic cells that can present exogenous antigens to naive or memory T cells, activating them

Clonal expansion

The mass proliferation of antibody-producing cells by clonal selection.

Activation of B cells

inactive B-cells have antibodies in their plasma membrane; if these antibodies match an antigen, the antigen binds to the antibody; an activated helper T-cell with receptors for the same antigen can then bind to the B-cell; the activated helper T-cell sends a cytokine signal to the B cell, causing it to change from an inactive to an active state

Clonal selection

The process by which an antigen selectively binds to and activates only those lymphocytes bearing receptors specific for the antigen. The selected lymphocytes proliferate and differentiate into a clone of effector cells and a clone of memory cells specific for the stimulating antigen.

Clonal deletion

The process of destroying B and T cells that react to self antigens

T-dependent antigen

An antigen that will stimulate the formation of antibodies only with the assistance of T helper cells

T-independent antigen

LPS or carbohydrates with repeating subunits which can activate B cells without TH cell help; produce weaker immune response involving IgM; no memory component generated

Antigen-antibody complex

The complex formed when an antibody binds to an antigen.

Thymic selection

Destroys immature T cells that don't recognize self-molecules of MHC

Microfold cells

(or M cells) Cells that take up and transfer antigens to lymphocytes, on Peyer's patches

Peyer's patches

large collections of lymphoid tissue found in the submucosa of the small intestine

Dendritic cells (DCs)

Engulf and degrade microbes and display them to T cells; found in skin, urogenital tract, lymph nodes, spleen, thymus and blood

Macrophages

Found within the lymph nodes, they are phagocytes that destroy bacteria, cancer cells, and other foreign matter in the lymphatic stream and present antigens to T cells

Clusters of differentiation (CD)

classification of T cells by the presence of certain glycoproteins on their surface

CD4+ T cells

T helper cells whose role is cytokine secretion; interact directly with antigens; bind MCH class II molecules on B cells and APCs

CD8+ T cells

Cytotoxic T cells; bind MHC class I molecules on body cells

TH activation

Foreign body is engulfed by APC and an antigen is presented on MHC II on APC; TCR on TH cell recognizes and binds the antigen-MHC II complex; APC or TH secretes costimulatory molecules to activate the TH

TH17 cells

CD4 T cells that produce IL-17 and contribute to inflammation

TH1 cells

CD4 T cells that secrete interferon gamma, which activates macrophages, enhances complement, and stimulates antibody production

TH2 cells

CD4 T cells that produce IL-4 and activate B cells to produce IgE; activate eosinophils

T regulatory cells (Treg)

Subset of CD4+ cells; carry an additional CD25 molecule

Suppress T cells against self; protect intestinal bacteria required for digestion; protect fetus

Cytotoxic T cells

Recognize and destroy non-self cells and self-cells altered by infection (carrying endogenous antigens on MHC I) by attaching to them and releasing perforin molecules, which form pores in the target cell membrane, and granzymes, which induce apoptosis

Endogenous antigens

Foreign antigens that are present inside body cells and presented on MHC I molecules to which CD8 can bind

Precursor cytotoxic T cells

CTLp; activated to CTLs with help of TH and costimulatory molecules

Apoptosis

Process of programmed cell death where cells fragment their genome and induce membrane blebbing; prevents spread of infection

Activation of cytotoxic T lymphocytes

Binding of TCR on TH cell to an abnormal antigen presented on MHC I on a self-cell; Activated TH produces cytokines to activate CD8 T cells; CD8 binds to abnormal antigen presented on MHC I and produces perforin and granzymes to kill the cell

Natural Killer (NK) cells

Granular leukocytes destroy cells that don't express MHC I such as virus-infected cells, parasites, and tumor cells; not always stimulated by antigen; forms pores in target cells resulting in lysis or apoptisis

Antibody-dependent cell-mediated cytotoxicity

Protozoans and helminths are too large to be phagocytized but can be coated with antibodies; Macrophages, eosinophils, and NK cells attach to Fc regions of antibodies coating the parasite and release chemicals to lyse the cells

Memory response

The rapid and enhanced immune response to a subsequent encounter with a familiar antigen due to memory cells; secondary response or anamnestic response; More rapid, lasts many days, greater in magnitude

Class switching

The process in which a plasma cell changes the type of antibody Fc region (stem) that it synthesizes and secretes; IgM first produced, switches to IgG during secondary response

Antibody titer

Amount of antibody in serum; reflects intensity of humoral response

Active immunity

A form of acquired immunity in which the body produces its own antibodies against disease-causing antigens.

Passive immunity

An individual does not produce his or her own antibodies, but rather receives them directly from another source, such as mother to infant through breast milk

Naturally acquired immunity

Immunity that occurs when a person has contracted and is recovering from a disease

Artificially acquired immunity

Immunity that results when antigens or antibodies are injected into an individual

Naturally acquired active immunity

Antibodies made due to direct exposure to an antigen during an infection

Naturally acquired passive immunity

Immunity conferred by transfer of maternal antibodies across placenta or in breast milk

Artificially acquired active immunity

Immunity in which antibodies are made due to exposure to a vaccine

Artificially acquired passive immunity

Immunity in which antibodies gathered from a human or other animal and are injected into an individual.

Live attenuated vaccines

Vaccine with a weakened pathogen; closely mimic an actual infection; confers lifelong cellular and humoral immunity; can be difficult to store and risk of transmission

Live attenuated vaccine examples

Measles, mumps, rubella, tuberculosis, varicella (chickenpox)

Inactivated vaccines

Pathogen has been completely killed or inactivated via heat, radiation, or chemicals; There is no active infection and it is easy to store; Weaker immunity results and it frequently requires boosters.

Inactivated vaccine examples

Influenza, plague, rabies, hepatitis A

Subunit vaccine

Vaccine containing only specific antigenic proteins of the infectious agent; low risk of side effects; limited longevity providing humoral immunity only that may require multiple doses; does not protect against antigenic variation

Subunit vaccine examples

Influenza, meningitis, papillomavirus, whooping cough

Toxoid vaccines

Chemically or thermally modified toxins used to stimulate active immunity; humoral immunity is generated, but does not protect against infection

Toxoid vaccine examples

Diphtheria, tetanus, botulism

Conjugate vaccines

Created by attaching bacterial capsule polysaccharides to an antigen; produces T-dependent response (better in children); expensive and provides no protection against antigenic variation

Conjugate vaccine examples

Meningitis

mRNA vaccines

mRNA delivered via lipid nanoparticles is translated in cells to generate antigen that can induce an immune response; no active infection and low risk of side effects; produces humoral and cell-mediated immunity; difficult to store and transport

mRNA vaccine examples

SARS-CoV-2