8.3 The Adaptive Immune System

Humoral Immunity

Involves the production of antibodies and may take as long as a week to become fully effective.

Antibodies

Produced by B-cells, they are specific to the antigens of the invading microbe.

Immunoglobulins

Another name for antibodies, they can attract leukocytes to phagocytize invaders, they can agglutinate which is when the pathogens clump together, or they can render invaders inactive by blocking their entry to the tissues.

Agglutination

Antibodies may cause pathogens to clump together to form large insoluble complexes that can be phagocytized.

Cell-Surface Antibodies

The binding of an antigen to a B-cell causes activation of that cell, resulting in its proliferation and formation of plasma and memory cells.

Degranulation

When an antigen binds to antibodies on the surface of a mast cell, it causes the cell to exocytose its granule contents, allowing the release of histamine and causing an inflammatory allergic reaction.

Structure of an Antibody Molecule

Variable Region Domain

The region at the tip of the Y that holds specific polypeptide sequences that will bind to only one antigen sequence.

Hypermutation

B-cells undergo this on its antigen-binding region, trying to find the best match for the antigen.

Clonal Selection

Mechanism of generating antibody specificity by allowing only B-cells with high affinity to their antigen to survive.

Constant Region

The part of the antibody molecule that cells like natural killers, macrophages, monocytes, eosinophils, and many others bind to.

Isotype Switching

The process by which cells can change which isotype of antibody they produce when stimulated by specific cytokines.

Naive B-Cells

When B-cells are released into the bloodstream when they have not yet been exposed to an antigen.

Plasma Cells

Produce large amounts of antibodies.

Memory B-Cells

When a B-cell remains in the lymph node, awaiting reexposure to the same antigen.

Primary Response

Initial activation of B-cells which takes approximately 7 to 10 days and results in forming memory B-cells to speed up future exposures.

Secondary Response

When an organism encounters a familiar antigen, the memory B-cells for that pathogen begin producing antibodies specific to that pathogen.

Vaccinations

Rely on the memory B-cells.

T-Cells

Mature in the thymus, where they undergo both positive and negative selection.

Positive Selection

Refers to maturing only the cells that can respond to the presentation of antigen on MHC. Cells that cannot respond to MHC undergo apoptosis because they will not be able to respond in the periphery.

Negative Selection

Refers to causing apoptosis in cells that are self-reactive, meaning they are activated by proteins produced by the organism itself.

Thymosin

A peptide hormone secreted by the thymic cells that drives T-cell maturation.

Naive T-Cell

A T-cell that has already matured in the thymus but has not yet encountered an antigen.

Helper T-Cells, Cytotoxic T-Cells, Suppressor T-Cells

The three major types of T-cells.

Helper T-Cells

Also known as CD4+ T-cells, they coordinate the immune response by secreting lymphokines.

Lymphokines

Released from helper T-cells, these molecules are capable of recruiting other immune cells such as plasma cells, cytotoxic T-cells, or macrophages and increasing their activity.

Lymphocyte Chemotaxis

Circulating lymphocytes can detect a very small concentration of lymphokine and then move up the concentration gradient towards where the immune response is required.

Interleukins

The most common family of lymphokines secreted by helper T-cells.

Human Immunodeficiency Virus

Infection by this virus causes the loss of helper T-cells, thus preventing the immune system from mounting a response to an infection.

CD4+ T-Cells

Another name for helper T-cells, they respond to antigens presented on MHC-II molecules. Since MHC-II molecules present exogenous antigens, these cells are most effective against bacterial, fungal, and parasitic infections.

Cytotoxic T-Cells or Cytotoxic T-Lymphocytes or CD8+ T-Cells

Cells that are capable of directly killing virally infected cells by injecting toxic chemicals that promote apoptosis in the infected cell. These cells respond to antigens on MHC-I molecules, meaning that they respond to endogenous antigens. They are most effective against viral infections.

Suppressor or Regulatory T-Cells

These cells also express CD4 but are different from helper T-cells in that they also express a protein called Foxp3. These cells help to tone down the immune response once infection has been adequately contained.

Memory T-Cells

Lie in wait until the next exposure to the same antigen. When activated, the response is much quicker and more robust the second time around.

CD4/8 Mnemonic

CD4+ T-cells are better at fighting extracellular infections. CD8+ T-cells are better at fighting intracellular infections.

(CD x MHC = 8); (4 x 2 = 8) and (8 x 1 = 8).

Lymphocytes of Specific Immunity

Ig types in humoral immunity: GAMED.

Th1 and Th2 Cells

The two types of CD4+ T-cells, which are activated when dendritic cells present the antigen to T-cells.

Th1 Cells

Release interferon gamma, which activates macrophages and increases their ability to kill bacteria.

Th2 Cells

Help activate B-cells and are more common in parasitic infections.

After Pathogen Elimination

The plasma cells die off but memory B-cells and memory T-cells remain and allow for a much faster secondary response upon exposure to the pathogen at a later time.

Interferons

Produced and secreted in response to viral infection. They reduce the permeability of nearby cells which makes it hard for viruses to invade them, and they also reduce the rate of transcription and translation in these cells.

Malaise, Muscle Ache, and Fever

Symptoms caused by interferons.

Virally Infected Cells

Present intracellular proteins on their surface in conjunction with MHC-I, almost always including viral proteins.

CD8+ T-Cell Response to Virally Infected Cells

Recognize the MHC-I and antigen complex as foreign and will inject toxins into the cell to promote apoptosis.

Self-Antigens

The proteins and carbohydrates present on the surface of every cell of the body. Under normal circumstances, they signal to immune cells that the cell is not threatening and should not be attacked.

Autoimmunity

A condition in which the immune system attacks cells presenting certain self-antigens.

Hypersensitivity Reactions

Include allergies and autoimmunity. Occurs when the immune system attacks certain antigens that are not inherently threatening. Examples include pet dander, pollen, and peanuts.

Negative Selection

Mechanism of Preventing B-Cell Autoimmunity

Negative selection process by which B-cells in the bone marrow which are responsive to self-antigens are killed while still in the bone marrow.

Glucocorticoids

Drugs for treating autoimmune diseases. They work through their potent immunosuppressive qualities.

Active Immunity

Immunization through which the immune system is stimulated to produce antibodies against a specific pathogen. The means of exposure may be natural or artificial.

Natural Exposure

Antibodies are generated by B-cells once an individual becomes infected.

Artificial Exposure

Usually done through vaccines, in which an antigen is injected and antibodies are produced. The antigen may be a weakened or killed form of a microbe, or it may be a part of a microbe's protein structure.

Passive Immunity

Results from the transfer of antibodies to an individual. The immunity is transient because only the antibodies, and not the plasma cells that produce them, are given to the individual. A natural example is the transfer of antibodies from a mother to the fetus through the placenta and in the breast milk.

Intravenous Immunoglobulin

Given to patients in cases of exposure to tetanus or rabies to prevent the pathogen from spreading.

Mechanism of Preventing T-Cell Autoimmunity

Negative selection process by which T-cells in the thymus that respond to and attack self-antigens are killed off and not allowed to mature.