what is immunity?
protection that one organism has in place against another organism
- elimination/control of offending org
- est of conditions to prevent organism causing problems down the road
what is the immune system?
what does the immune system seek to target?
what are major challenges in achieving this goal?
network of many diff proteins, cells, tissues, organs
- highly regulated interations between all of them in order to...
1. target pathogens (disease-causing organisms: bacteria, viruses, fungi, parasites, prions)
2. target cancers (abberrant, uncontrolled cell growth due to gene mutations, oncogenes)
goal: coordinated attack against offending org
challenge: tricky to target pathogens and cancers bc they are numerous and largely made of the same stuff as us
what is autoimmunity?
what is tolerance?
immune system is constantly parsing out which cells are self and non-self
if it mistakes self cells as non-self, it can trigger an immune response against normal cells of the body via autoimmunity
tolerance: ability of the immune system to recognize and tolerate self cells
- loss of tolerance → autoimmunity!
- ex. Type 1 diabetes (immune system targets beta cells of Islets of Langerhaans)
- non-pathogen antigens
- substance that elicits an immune response
- most are proteins or polysacchs; often components of invading microbe (capsule, flagella, cell wall, toxin)
the reactive (structurally conserved) portion of the antigen (epitope) interacts with antibodies → antigen-antibody complex or immune complex
antigens can also be overexpressed, misexpressed, mutated proteins (i.e. cancer!)
antigen receptor diversity
diff antibodies can have diff antigen receptors for the same antigen (bind to diff epitopes)
secreted version of B cell antigen receptors!
bivalent: two heads (identical on a given ab)
comprised of 2 heavy chains, 2 light chains with variable and constant regions
- antigen binds to region of ab formed by association of variable light/variable heavy chains
variable regions : diff for every ab
- FR 1-4 : framework regions, imp for protein folding (without which, ab won't fold right, won't work)
- HV 1-3 : hypervariable regions,
constant region: similar for every ab
antibodies and relation to B cells and T cells
B cells: antibodies are secreted version of B cell receptors
- B cells will differentiate into plasma cells, which make antibodies
T cells: similar structure to antibodies and BCRs except
- always membrane-bound
- not bivalent (single head; alpha/beta instead of light/heavy)
how does tremendous antibody diversity come about?
generation of TCR and BCR genes via V to J somatic recombination
instead of typical exon/intron setup, TCR and BCR genes are present as gene fragments in the germline
- V alpha fragments - variable - approx 60
- J alpha fragments - junctional - approx 50
- C alpha fragments - constant - 1
as T cells and B cells mature, V to J somatic recombo occurs...
1. double stranded break in V section and in J section, intervening DNA discarded
2. fusion of gene segments : "messy", involves random addition/deletion of random nucleotides (diversity!!!)
types of tolerance
1. central tolerance
- removal of self-reactive clones
- occurs in thymus (T cells) and bone marrow (B cells)
2. peripheral tolerance
- ignorance : hiding self antigens from immune system to shut down autoimmune response
- anergy : shut down self-reactive clones
- suppression : use of other molecules/proteins/cells to keep self-reactive clones in check
immune system "memory"
antibodies and effector T cells persist for weeks after an infection to confer "protective immunity"
another exposure to the same antigen will produce a much faster, bigger, better response via "immunological memory"
part of circ system comprising lymphatic vessels that carry clear lymph
- contains immune cells (T cells, B cells, dendritic cells)
- filtered by lymph nodes before returning to circ
tissues and organs of lymphatic system are designed to bring antigen into contact with lymphocytes
- lymph nodes and spleen are key sites for immune system activation
common progenitor of immune system cells
hematopoeitic stem cells
→ lymphoid precursors (T cells - thymus, B cells - bone marrow, NK cells)
- helper T cell
- suppressor T cells
- cytotoxic T cell
- B cell → plasma cell
→ myeloid precursor
- mast cell → macrophage
cluster of differentiation #
- cell membrane molecules are used to classify leukocytes into subsets
- CD# is defined or classified by reference monoclonal antibodies to which they bind
ex. helper T cells (CD4 T cells), killer T cells (CD8 T cells), Tregs (CD25)
once mature, T cells migrate to lymphoid organs and wait to make contact with antigens
- once contact is made, they can kill infected cells and help activate other cells (B cells, macrophages)
form basis of cell-mediated immunity
antibody-producing cells that account for humoral immunity
- abs can provide protective immunity for decades
develop in bone marrow, often acquire specific functions later in the tissues
myeloid cells lumped together as mononuclear phagocytic system
- secretion of cytokines
- antigen presentation
how is the adaptive immune response triggered?
cells have to be tagged to get the adaptive immune response rolling → antigen presentation
tags in mice: MHC (major histocompatibility complexes)
tags in humans: HLA (human leukocyte antigens)
- pathogens are ingested and "tags" are displayed on membranes of macrophages or dendritic cells
- helper (CD4) and cytotoxic (CD8) recognize, proliferate, launch adaptive response
MHC/HLA Class I antigen processing pathway
virus enters a cell, duplicates
- viral protein is chopped up via proteasome → shuttled into ER, where it combines with Class I MHC/HLA → shuttled to Golgi and out to cell membrane
- CD8 killer T cells can recognize, prolif, destroy these cells
***MHC/HLA I mols are loaded with peptide made inside the cell → killing of infected target cells, activation of macrophages
MHC/HLA Class I molecules have a groove (peptide-binding cleft)
MHC/HLA Class II antigen processing pathway
bacteria enters a cell, shuttled to endosome
- bacterial protein is chopped up via enzymes in endosome, combines with Class II MHC/HLA → shuttled out to cell membrane
- CD4 helper T cells can recognize, prolif, assist in destruction of these cells
MHC/HLA II mols are loaded with peptides taken up by phagocytosis → activates macrophages, B cells, others; inflammatory response
capable of destroying other cells, esp virus-infected cells and tumor cells; also parasites
do not express TCRs or BCRs
important application: some pathogens downregulate MHC; NK cells can get these cells that T cells can't
"specific" defenses vs. "nonspecific" defenses
B cells defend against antigens/pathogens in body fluids : humoral immunity
T cells defend against abnormal cells, pathogens inside living cells : cell-mediated immunity
immunologist's dirty little secret
"clean" antigens will not activate immune system particularly well
- must be mixed with another substance that is not antigenic in and of itself, but which will potentiate the immune response: adjuvant
ex. mycobacterium in mice
components of pathogens are recog'd by "pattern recognition receptors" (germline-encoded receptors that bind path-specific mols like LPS)
- these receptors have to be activated before adaptive response can occur!
fluorescence activated cell sorting via flow cytometry
- can measure granularity and cell size → indicative of amount and type of cells present
- can use fluorescent tags to mark diff cell types and then quantify them using FACS
generation of monoclonal antibodies
inject antigen+adjuvant into an organism, wait for the immune response to occur, then isolate the spleen
fuse B cells with immortalized myeloma cell line (to keep them alive) and put in an HAT medium
- HAT will kill unfused myeloma cells; unfused B cells will die off by themselves after a few days
screen to see which B cells actually make the antibody of interest and now you have a hybridoma producing a monoclonal antibody!!!