Adaptive Immunity Flashcards
Transition from innate –> adaptive immunity
Occurs when antigen-presenting cells interact with a T or B cell that is specific for a given antigen
What is an antigen?
- Any molecule to which an antibody can specifically bind
- Generates antibodies
- Can be proteins, carbohydrates or nucleic acids
What is an antibody?
- Effector molecules produced by B cells that is specific for a given antigen
- When an Ab binds to it’s antigen, it tags the pathogen for destruction by phagocytes (like macrophages & B cells)
Antigen presentation
- T cells can’t recognize “native” antigen-only processed fragments of antigen
- APCs pick up antigens, process them & then present it on MHC proteins
- MHC proteins were discovered by failed tissue transplantations
- MHC1 is expressed on almost all nucleated cells & MHC2 is only expressed on professional APCs (macrophages, dendritic cells & B cells)
When/where do APCs process antigens?
- APCs begin processing antigens right after they have engulfed a pathogen
- They will then travel through the lymphatics to a lymph node (or through blood to the spleen) where they will present the antigens to naive T cells
T cell activation in the lymph nodes
- Each T cell has only one specificity of T cell receptor
- A T cell receptor will only react with one antigen
Generation of diversity
- Germline theory
- Separate gene for each distinct receptor - Somatic diversification theory
- Recombination of a limited number of gene sequences leading to a large number of distinct receptors
Clonal selection
A hypothesis which states that an individual lymphocyte (specifically, a B cell) expresses receptors specific to the distinct antigen, determined before the antibody ever encounters the antigen
B and T cell development
- Some will be auto reactive & target self molecules
- Both T and B cells undergo NEGATIVE selection to eliminate these cells (apoptosis)
- T cells also undergo POSITIVE selection; needs to be able to recognize our own MHC or else its useless
- Reduces chance of autoimmunity
T cell development
- T cell development occurs in the thymus
- T cell precursors travel from the bone marrow –> thymus to develop
- Mature T cells leave the thymus & travel to secondary lymphoid tissues (where they can surveil for APCs)
Types of T cells (x2)
CD4+ “helper” T cells
- They activate macrophages to kill the bacteria they have inside
- Activate T cells to produce antibodies
- Recruit neutrophils
CD8+ “killer” T cells
- They find infected cells and cause the cells to undergo apoptosis
Activation of B cells
- Antigen recognition induces expression of effector molecules by the T cell, which activates the B cell
- Differentiation to resting memory cells & antibody-secreting plasma cells
- Produce massive amounts of ER to produce lots of antibodies
B cells can sometimes be activated without T cells
- Activation when their receptors are cross-linked by antigens
- Receptors act kinda like an antibody
- Alone this is a weak signal, but if you bring them together –> looks like a strong signal
- Even things that might not be able to bind to a T cell receptor, can sometimes activate a B cell aka a T cell independent antigen
- T cell independent antigens do not make as good as of an immune response as a T cell dependent antigen
Functions of antibodies
- Neutralization: coat the pathogen with antibodies so the pathogen can’t bind to the epithelium
- Opsonization: antibody binding activating the complement cascade; promotes phagocytosis into a macrophage for killing
- Complement activation: if an antibody binds, the complement system is activated
- Antibodies can also bind to the surface of one of our cells
- Fc receptors on NK cells recognize bound antibodies
- Causes NK cells to release its toxic granules –> apoptosis (doesn’t release the virus particles, preventing spread)
- Now the virus infected cell is gone
Types of antibodies (x4)
- IgG
- Workhorse of the antibodies, does everything, immune memory, found almost everywhere, provides immunity to fetuses
- IgA
- Dimer, mucosal antibody, secreted out into lumen of respiratory/GI tracts, doesn’t go to fetus
- IgM
- First antibody produced (in primary immune response), found in blood, very big = doesn’t cross in or out of blood
- IgE
- Associated with allergy & parasite defence (mast cells have IgE), found at body interfaces - found under skin –> hives & itchy skin
NOTE: dimeric IgA makes its way into the breast milk
- Breastfed infants benefit from mom’s IgA that they can now take into the lumen of their GI tract
