antigens and antibodies Flashcards
antigen
molecule that can induce an antibody response
immunogen
molecule that induces an immune response (usually protein or large, multivalent non-protein)
hapten
an antigen (such as a small chemical) that can bind antibodies but does not induce an immune response on its own
compare the body’s response to first exposure of a hapten vs polysaccharide vs protein antigen
hapten: antibody binding but no B or T cell activation
polysaccharide: antibody binding, B cell activation, no T cell activation
protein: antibody binding and B/T cell activation
how can an immune response to a hapten be generated?
link multiple copies of a hapten to a protein carrier:
- T cell response to hapten and to carrier protein
- T cell dependent B cell response to carrier protein => antibodies specific to hapten
heavy chain
Mu, delta, gamma, alpha, or epsilon constant region that determines the antibody isotype
light chain
either kappa or lambda
Fc
- contains constant region and complement binding region
- many phagocytes have receptors that bind this region => enhances phagocytosis (opsonization)
- complement binding region is only accessible when antigen is bound
Fab
- antigen binding fragment
- contains variable regions and complementarity determining regions
isotypes
defined by differences in aa sequences in the constant regions (ex: IgM vs IgG)
allotypes
defined by differences between individuals due to genetic polymorphisms (ex: IgM in two different people)
idiotypes
defined by specific aa’s in the hypervariable region
IgM
- mu heavy chain
- fixes complement
- expressed on surface as BCR
- first antibody produced in primary response (and in T cell independent)
- pentamer structure => increased avidity
- does not cross placenta
IgD
- delta heavy chain
- expressed on naive B cell surface with IgM
- function not understood
IgG
- gamma heavy chain
- most effective for many infections: opsonizes bacteria, fixes complement, neutralizes bacteria and toxins
- most abundant in secondary response
- crosses placenta
IgE
- epsilon heavy chain
- antibody dependent cellular cytotoxicity: binds extracellular parasites and mediates killing by eosinophils
- makes mast cells release histamine => allergy and anaphylaxis
IgA
- alpha heavy chain
- mucosal immunity: prevents attachment of virus/bacteria to mucosal membranes
- does not fix complement
- passively transferred to infants in breast milk
- monomeric in serum; dimeric at mucosa
how do IgG and IgM enhance opsonization
IgG:
- classical path of complement activation (form C3b)
- direct binding by IgG Fc receptors on phagocytes
IgM:
- classical path of complement activation
what is class switching and how is it induced?
in a T cell dependent immune response, IgM, IgG, IgA, and IgE can be produced:
- class switch depends on the cytokines released by CD4+ T helper cells (IL-4 => IgG and IgE; IL-5 => IgA
- requires CD40/CD40L binding
- activation-induced cytidine deaminase is induced and mediates switch
steps in a T cell dependent immune response
- surface IgM BCR recognizes a protein antigen
- B cells express peptides on MHC class II and present to T cells
- B cell CD40 - T cell CD40L interaction => germinal center formation, class switching, and somatic hypermutation
- high affinity antibodies produced
steps in a T cell independent immune response
- B cells recognize a non-protein antigen with repeated identical antigenic epitopes
- Crosslinking surface IgM activates the B cell via complement (CD21) and TLRs
- Production of lower affinity secreted IgM specific for antigen (no affinity maturation; limited memory)
*splenic and mucosal B cells
natural antibodies
- IgM present prior to exposure, possibly due to T cell-independent response to environmental antigens
- example: antigens against ABO glycoproteins on blood
