Chapter 12 Microbiology

Name the different classes and subclasses of T cells, state how they're differentiated from one another, and describe their general roles in adaptive immunity.

Two main types: 1) Helper T cells - activate macrophages to kill phagocytosed microbes 2) Cytotoxic T cells - kill infected cells and eliminate reservoirs of infection -TH cell help in humoral immunity 1. TH cell binds with the self-nonself complexes of a B cell that has encountered its antigen and is displaying it on MHC II on its surface. 2. TH cell releases interleukins as costimulatory signals to complete B cell activation. -TH cell help in cell-mediated immunity 1. Dendritic cell engulfs an exogenous antigen, processes it, and displays its fragments on class II MHC protein. 2. Immunocompetent CD4 cell recognizes antigen-MHC complex. Both TCR and CD4 protein bind to antigen-MHC complex. 3. CD4 cells are activated, proliferate (clone), and become memory an effector cells

Define immunological memory and compare it to a primary response.

Immunological memory: ability of the immune system to produce a faster and more efficient immune response upon subsequent exposure to a pathogen previously encountered. Memory responses, which are called secondary, tertiary, and so on, depending on the number of exposures to antigen, also differ qualitatively from primary responses. This is particularly clear in the case of the antibody response, where the characteristics of antibodies produced in secondary and subsequent responses are distinct from those produced in the primary response to the same antigen.

How does adaptive immunity differ from innate immunity?

Innate immunity is something already present in the body. It is non-specific and fights any foreign invader Adaptive immunity is created in response to exposure to a foreign substance. It is specific and fights only the certain infection. Think of the word adaptation, this type of immunity has to adapt to the invading pathogen in order to create a defense against it. Therefore, this immunity takes longer (innate is rapid response) but it more potent than innate.

What is isotype switching and how is it advantageous?

Isotype class switching is a biological mechanism that changes a B cell's production of antibody from one class to another; leads to individual B cells that produce antibodies with the same antigen specificity but with different functions; expanded antibody response as different isotypes predominate in different areas of the body. During this process, the constant region portion of the antibody heavy chain is changed, but the variable region of the heavy chain stays the same. Since the variable region does not change, class switching does not affect antigen specificity. Instead, the antibody retains affinity for the same antigens, but can interact with different effector molecules.

How do MHCs impact transplant rejection?

MHC molecules responsible for stimulating the rejection of tissue grafts and organ transplants; it is necessary that the MHCs of the donor and the recipient be as similar as possible.

What are TCRs and BCRs, what role do they play in antigen recognition, and how does their diversity come about?

•Functions: recognize antigens derived from pathogens. •display their diversities and similarities with respect to recognition of types of antigens, receptor functions, and signal transduction. Diversity 1: B and T cell receptors are differ in recognition of antigen structures •B Cell Receptors (BCRs) are of two types: Membrane bound and secreted (called antibodies)T cell receptors (TCRs) are of only one type, membrane bound. They do not have any 'secreted' versionTCRs recognize: peptides together with MHC molecules on APCs, viral-infected cells and tumor cells.

How are T cells activated and what factors affect T cell subclass differentiation?

-The two-signal activation process for T cells includes interaction of the T-cell receptor with the MHC-antigen complex on the APC AND binding between co-stimulatory proteins on the T cell and APC surfaces. -T cell subclass differentiation is caused by the release of cytokines that is influenced by the antigen's nature and the amount present.

What are the basic structural and functional features of antibodies?

1) increase phagocytosis by agglutination of antigens. 2) neutralize antigens to prevent binding to host cells. 3) increase phagocytosis by opsonization. 4) activate the complement cascade.

Name the five antibody isotypes and provide details on their structures and functions.

1. IgG (secreted by plasma cells in the blood, able to cross the placenta into fetus) 2. IgA (found in mucous, saliva, sweat, tears, and breast milk. Protects against pathogens) 3. IgM (may be attached to the surface of a B cell or secreted into the blood. responsible for early stages of immunity) 4. IgE (protects against parasitic worms. responsible for allergic reactions) 5. IgD (part of B cell receptor. Activates basophils, and mast cells)

Name the four categories of adaptive immunity, provide an example of each, and state which ones generate long-term immunity and why they can do so.

Active immunity Immunity derived from antibodies generated by own body. Generated in response to normal infection or in response to vaccine antigens. Passive immunity Immunity derived from antibodies from another body, such as given through mother's milk or artificial means (antivenom antibodies). Generally does not last as long as active immunity as antibodies are not supplied by the body. Natural immunity Antibodies generated due to a normal infection. Includes antibodies provided by mother through their colostrum, the initial breastmilk. Only protects for short time, but when infant is most vulnerable. Artificial immunity Antibodies generated by receiving antigen in a vaccine/serum rather than a normal infection. Generally does not last as long as when generate antibodies from a natural infection.

What's an antibody titer and how would a secondary exposure alter it?

An antibody titer is a type of blood test that determines the presence and level (titer) of antibodies in the blood. This test is carried out to investigate if there is an immune reaction triggered by foreign invaders (antigens) in the body. a second exposure to antigen produces a much faster response, and several orders of magnitude higher levels of antibody. The ability of antibody to bind antigen also increases in the secondary response. The memory of antigen and stimulated response is the basis for vaccination.

Compare and contrast the main lymphocytes of adaptive immunity.

B cells and T cells are the major types of lymphocytes involved in adaptive immunity. B and T cells can create memory cells to defend against future attacks by the same pathogen by mounting a stronger and faster adaptive immune response against that pathogen before it can even cause symptoms of infection. B cells do not attack pathogens themselves but instead produce antibodies that attack the pathogens or direct other cells, such as phagocytes, to attack them. B cell mechanisms are therefore called "antibody-mediated immunity". Because antibodies disperse freely in blood, this type of immunity is sometimes also called "humoral immunity". Humoral = body fluids, especially blood plasma. T cells attack pathogens more directly. T cell immune mechanisms are classified as "cell-mediated immunity", or more simply, "cellular immunity".

How do T cytotoxic and T helper cells eliminate and remember antigens?

Cytotoxic T cells eliminate pathogens by releasing chemicals which cause infected or cancerous cells to undergo apoptosis.

What do the terms epitope, antigen, hapten, and immunogenicity mean?

Epitope: also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. For example, the epitope is the specific piece of the antigen to which an antibody binds; stimulate immune response. Hapten: small molecule that elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself. (can bind to antibodies or B-cell receptor) Immunogenicity: the ability of a foreign substance, such as an antigen, to provoke an immune response in the body of a human or other animal.

What antigens tend to be most immunogenic?

Generally an antigen has several or many different epitopes and reacts with many different antibodies, an epitope is approximately five or six amino acids in length. Those antigens that are capable of inducing an immune response are said to be immunogenic and are called immunogens.

What are the main types of MHCs and how do they present antigens to T cells?

MHCs. 1.) Class 1 MHC (MHC I): found on all body cells except red blood cells (RBCs) and presents a sample of cellular proteins, including those of any intracellular pathogens, to T cells. 2.) Class 2 MHC (MCH II): made by dendritic cells, macrophages, and B cells. MHC II interacts with both the CD4 and CD8 receptors on T helper and T cytotoxic cells.

Define self-tolerance and state how B and T cells are screened for this capability; include a discussion of MHCs in self-tolerance screening.

Self-tolerance: Self-tolerance is the ability of the immune system to recognize self-produced antigens as a non-threat while appropriately mounting a response to foreign substances. This balance of immunological defense and self-tolerance is critical to normal physiological function and overall health. T cells are screened for self-tolerance in the thymus. B cells mature and are screened for self-tolerance in the bone marrow. Self-tolerance is what prevents lymphocytes from attacking the body's own cells. Without self-tolerance, a person develops autoimmune disorders. The major histocompatibility complex (MHC) genes code for proteins which the immune system uses to identify cells and tissues in the body as "self" or "other". MHC molecules 'talk' to T cells which patrol the body for foreign invaders or dangerously mutated cells. The MHC acts as a window into our cells. It presents snippets of information (peptides) on the state of the cell- allowing the immune system to check for infection, cancer, and other maladies. Cells that do not pass the self/other test are eliminated.

How do superantigens activate T helper cells?

T cell activation by superantigens are not processed and presented by APCs and cause nonspecific activation of many T cells at once, including those that would not normally recognize the antigen.

Compare and contrast T-dependent and T-independent antigens and their modes of activating B cells.

T-independent antigens are usually polysaccharides and able to bind multiple B cell receptors on a given B cell.

What is the cellular response and what are its key effector cells?

The cellular branch of adaptive immunity is mainly organized by T helper cells and carried out by T cytotoxic cells. Effector cells are activated cells - Effector B cells are plasma cells that produce antibodies - Effector T cells are Cytotoxic T cells and Helper T cells

What are the branches of adaptive immunity?

There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity. Humoral immunity involves the production of antibodies and cell-mediated immunity involves the production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells, and cytokines.