Adaptive (Acquired) Immunity

Question 1

Adaptive immunity

Answer 1

AKA acquired immunity; uses specific antigens to strategically mount an immune response, activated by exposure to pathogens and uses immunological memory to learn about the threat and enhance the immune response accordingly; much slower to respond to threats and infections than innate immunity

Question 2

What are the cells involved in adaptive immunity?

Answer 2

This specific immune response relies on B and T cells to carry out the tasks (lymphocytes derived from stem cells in the bone marrow)

Question 3

B cells

Answer 3

Cells that form and mature within the bone marrow; they have membrane-bound antibodies
B cell receptor is an IgM monomer; antigen binds to B cell receptor and is internalized for initiation of antibody production (T-independent B cell activation) not requiring helper T cells; slower response (humoral immunity)

Question 4

Humoral immunity

Answer 4

B cells and the antibodies they produce; more rapid response

Question 5

Types of B cells

Answer 5

Only B cells secrete receptors; in the form of antibodies; Naïve B cells multiply and differentiate into either memory or plasma cells
- Plasma cells: secrete large amounts of antibodies
- Memory cells: circulate and remember a specific antigen so that if it returns, the response will be quicker

Question 6

Types of antigen-triggered B cell activation:

Answer 6

1. T-dependent: requires helper T cells to “help” activate the B cell; evoked by proteins; macrophages ingest and process antigen, then present it to helper T cells; then helper T cells activate B cells; strong response including the production of memory cells
2. T-independent: does not require T helper cells; usually evoked by large molecules with repeating units like polysaccharides; slower response with no memory cells produced (no long term immunity here)

Question 7

T cells

Answer 7

T progenitor cells form in the bone marrow and then migrate to the thymus where they express T cell receptors (TCRs) and other receptors (CD4 and CD8)
TCRs can only recognize antigens that are bound to certain receptor molecules called MHC class 1 and class 2
MHC molecules are membrane-bound surface receptors on antigen-presenting cells (APCs, like dendritic cells and macrophages)
CD4 and CD8 play a role in T cell recognition and activation by binding to either MHCI or MHCII
T cell receptors undergo rearrangement which leads to limitless recombination of genes (lots of binding diversity)

Question 8

T cell selection processes:

Answer 8

1. Positive selection: ensures MHC restriction by testing MHCI and MHCII ability to distinguish between self and non-self proteins; must be capable of binding only self-MHC molecules
2. Negative selection: tests for self-tolerance; tests binding abilities of CD4 and CD8 specifically; T-cell should only bind to self-MHC molecules presenting a foreign antigen

Question 9

Types of T cells:

Answer 9

Helper T cells express CD4 and help with activation of cytotoxic T cells, B cells, and other immune cells
Cytotoxic T cells express CD8 (remove pathogens and infected host cells)
T regulatory cells express CD4 and CD25. help distinguish between self and nonself molecules (reduce risk of autoimmune disease)

Question 10

Antigens

Answer 10

any substance (usually foreign) with the ability to stimulate an immune response when presented in an effective fashion (usually proteins, polysaccharides, glycolipids)
Large, complex antigens such as proteins and viruses must be processed before their epitopes can be effectively recognized by the immune system
Macrophages and specialized epithelial cells in the skin and lymphoid organs do this by ingesting the antigen, degrading it into smaller pieces, and presenting those pieces on major histocompatibility molecules (MAC) on their cell surface to be recognized by T cells

Question 11

Epitopes

Answer 11

subregion of an antigen that is the actual antigenic determinant (site where antigen binds) fit the combining site of T cell receptors and antibodies
T cell receptors recognize protein epitopes
B cell receptors recognize proteins, lipopolysaccharides, polysaccharides, and carbohydrate antigens
Antigens can have multiple epitopes for both B and T cells; larger antigen = more epitopes available

Question 12

Antigen-presenting cells (APCs)

Answer 12

present antigens to activate T cells;
Goal is to not kill pathogens in large numbers but to phagocytize small numbers; do not activate complement or attract neutrophils

Question 13

Antibodies

Answer 13

Specialized Y-shaped proteins that tag antigens for destruction; belongs to the immunoglobulin family of proteins; present in the blood, the surface of B cells, and other body fluids

Question 14

Antibody structure

Answer 14

• Composed of 2 light polypeptide chains and 2 heavy polypeptide chains
• Bound by disulfide bonds (separated by a reducing agent)
• Y shaped with 2 antigen binding sites (Fab fragment) (variable region)
• Variable region: part that specifically binds to the antigen
• Stalk is called the Fc fragment; phagocytes have Fc receptors and antibodies “tag” antigens for removal by phagocytes; Fc region contains constant region that does not bind to any antigens

Question 15

Structural Types of Antibodies

Answer 15

IgG- defense against infection
IgA- defense against infection
IgM- defense against infection
IgE
IgD

Question 16

IgG

Answer 16

monomer; most abundant in the body and provides the most extensive and long-lived antibody response; memory cells are programmed for rapid IgG response upon a repeat infection (lifelong immunity); neutralizes bacterial exotoxins and viruses usually by blocking their attachment to cell receptors; only immunoglobulin able to cross the placental barrier; take about 1 month to peak

Question 17

IgA

Answer 17

monomer in blood; dimer connected at the Fc region in the gut, tears, other secretions
Special role in local immunity in protecting epithelial surfaces from colonization and infection; sIgA is present on mucosal layers on epithelial surfaces and in glandular secretions; prevents attachment of potential pathogens to receptors on mucous membrane epithelia

Question 18

IgM

Answer 18

pentamer (5); monomers are present on the surface of B cells in the form of B cell receptors; pentameric IgM in the blood is very good at agglutinating particles because of its many binding sites; works closely with the complement system; peak in the serum after about 2 weeks

Question 19

Antibody production timeline:

Answer 19

initial contact with a new antigen evokes the primary response; lag phase of about 1 week between the challenge and the detection of antibodies (B cells produce antibody); levels of antibody rise to a maximal steady state in about 3 weeks; antibody levels then decline gradually if no further stimulation is given; first antibodies synthesized are IgM, then IgG arises and eventually predominates
After a subsequent exposure or booster injection of the same antigen, secondary response occurs: involves memory, lag time is shortened so the rate of exponential increase to maximum steady-state level is more rapid, steady-state level is higher; mostly IgG is formed; IgG antibodies formed have a higher affinity to their antigen

Question 20

Cell-mediated immunity

Answer 20

acts on intracellular microbes (bacteria, viruses, parasites, tumor cells) it is a delayed type of hypersensitivity reaction
macrophages are mobilized and enhanced
- Helper T cells: facilitate the organization of the immune response; releasing messenger substances, stimulating production of antibodies against specific pathogen, and disperse the antibodies throughout the bloodstream
Controlled by a collection of genes called the major histocompatibility complex (MHC); codes for molecules are present on the surface of almost all human cells

Question 21

MHC I

Answer 21

present on almost all human cells; marks cell as “self”
binds to products generated inside the cell (ex. Viral infection); viral proteins are digested to peptides in proteasomes in the cytoplasm and attached to MHC I molecules in the endoplasmic reticulum where they are then transported to the surface for presentation; recognized by CD8+ T cells

Question 22

CD8+ helper T cells

Answer 22

kill host cells presenting foreign antigens on MHC class I molecules; prevent viral production and release by eliminating the host cell before viral synthesis or assembly is complete

Question 23

MHC II

Answer 23

binds to products that originated outside of the cell, but have been phagocytized and are now in a vacuole inside the cell; product is digested and peptide fragments are combined with MHC II molecules and transported to the surface for presentation; recognized by CD4+ T cells

Question 24

CD4+ helper T cells

Answer 24

stimulated and activated by antigens presented on MHC class II molecules on the surface of host cells; upon activation, the T cell differentiates and divides: Th I, Th 2 (dif. Cell-mediated activities) Th17 (ILs, stimulate neutrophils, effective against extracellular bacteria and fungus)