antigens and antibodies

Question 1

antigen

Answer 1

molecule that can induce an antibody response

Question 2

immunogen

Answer 2

molecule that induces an immune response (usually protein or large, multivalent non-protein)

Question 3

hapten

Answer 3

an antigen (such as a small chemical) that can bind antibodies but does not induce an immune response on its own

Question 4

compare the body’s response to first exposure of a hapten vs polysaccharide vs protein antigen

Answer 4

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

Question 5

how can an immune response to a hapten be generated?

Answer 5

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

Question 6

heavy chain

Answer 6

Mu, delta, gamma, alpha, or epsilon constant region that determines the antibody isotype

Question 7

light chain

Answer 7

either kappa or lambda

Question 8

Fc

Answer 8

• 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

Question 9

Fab

Answer 9

• antigen binding fragment

- contains variable regions and complementarity determining regions

Question 10

isotypes

Answer 10

defined by differences in aa sequences in the constant regions (ex: IgM vs IgG)

Question 11

allotypes

Answer 11

defined by differences between individuals due to genetic polymorphisms (ex: IgM in two different people)

Question 12

idiotypes

Answer 12

defined by specific aa’s in the hypervariable region

Question 13

IgM

Answer 13

• 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

Question 14

IgD

Answer 14

• delta heavy chain
• expressed on naive B cell surface with IgM
• function not understood

Question 15

IgG

Answer 15

• gamma heavy chain
• most effective for many infections: opsonizes bacteria, fixes complement, neutralizes bacteria and toxins
• most abundant in secondary response
• crosses placenta

Question 16

IgE

Answer 16

• epsilon heavy chain
• antibody dependent cellular cytotoxicity: binds extracellular parasites and mediates killing by eosinophils
• makes mast cells release histamine => allergy and anaphylaxis

Question 17

IgA

Answer 17

• 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

Question 18

how do IgG and IgM enhance opsonization

Answer 18

IgG:

• classical path of complement activation (form C3b)
• direct binding by IgG Fc receptors on phagocytes

IgM:
- classical path of complement activation

Question 19

what is class switching and how is it induced?

Answer 19

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

Question 20

steps in a T cell dependent immune response

Answer 20

1. surface IgM BCR recognizes a protein antigen
2. B cells express peptides on MHC class II and present to T cells
3. B cell CD40 - T cell CD40L interaction => germinal center formation, class switching, and somatic hypermutation
4. high affinity antibodies produced

Question 21

steps in a T cell independent immune response

Answer 21

1. B cells recognize a non-protein antigen with repeated identical antigenic epitopes
2. Crosslinking surface IgM activates the B cell via complement (CD21) and TLRs
3. Production of lower affinity secreted IgM specific for antigen (no affinity maturation; limited memory)

*splenic and mucosal B cells

Question 22

natural antibodies

Answer 22

• IgM present prior to exposure, possibly due to T cell-independent response to environmental antigens
• example: antigens against ABO glycoproteins on blood