1. Immunoglobulins structure and function

2. IMMUNOGLOBULINS Definition Glycoprotein molecules that are
present on B cells (BCR) or produced by plasma cells (usually referred to as antibodies) in response to an immunogen

3. Antibodies production is the sole function of the B cells
Not toxic or destructive, bind the pathogen tightly and target destructive components of the immune system
Antibodies are useful in the defense against extracellular pathogens
Antibodies are secreted in the secondary lymphoid organs and in bone marrow and find their way to the extracellular spaces
During the course of an infection antibody effectiveness improves steadily

4. PHASES OF B CELL RESPONSE!
PHASES OF B CELL RESPONSE
Gene rearrangement in an immature B cell leads to the expression of membrane-bound IgM and IgD on the mature B cell surface. After encounter with an antigen this isotype is produced in a secreted form by the plasma cell (IgM largely while IgD only is small amounts). All isotypes (IgA, IgD, IgE, IgG and IgM) can be made in two forms: the bounded immunoglobulins serving as BCRs, and soluble secreted antibodies by the plasma cells. (The difference b/w membrane bound and secreted is a hydrophobic part and hydrophilic parts at the carboxy terminus, respectively).

5. Immunoglobulin STRUCTURE!
Immunoglobulin STRUCTURE
2x Heavy chain (light blue)
2x light chain (dark blue)
Variable regions  antigen binding
Constant regions
disulfide bond
carbohydrate CL VL
CH2
CH3
CH1 VH
hinge region

6. Ribbon structure of IgG

7. ! ! BCR (B cell receptor) Antibody
Transmembrane domain
Associated chains for signaling
Cytoplasmic domain
SOLUBLE (freely circulating)
MEMBRANE BOUND!
Antigen recognition and effector functions.
Produced by plasma cells
Antigen recognition and B cell activation

8. ! ! ANTIBODY DOMAINS AND THEIR FUNCTIONS Variable domain
Antigen recognition
Variable domain
Constant domain
Effector functions

9. Associated chains providing signaling capacity!
mIg = BCR
Associated chains providing signaling capacity

10. !
B CELL ACTIVATION
B cell
BCR oligomerization results in B cell activation, proliferation and differentiation

11. FLEXIBILITY OF ANTIBODIES

12. FEATURES OF ANTIBODY-ANTIGEN INTERACTION
Valency: numbers of antigen epitopes an antibody binds
Affinity: the strength of interaction between a specific antigen and one binding site of the antibody
Avidity: The overall strength of binding at multiple sites in an antibody

13. ANTIGEN BINDING Antigen Binding Fragment (Fab) Complement binding site
Placental transfer
Constant fragment (Fc)
Binding to Fc receptors on phagocytic cells

14. Idiotype VARIABILITY IN DIFFERENT REGIONS OF THE Ig
DETERMINES Ig SPECIFICITY OR CLASS
isotype
Idiotype
Sequence variability of H/L-chain constant regions
Sequence variability of H/L-chain variable regions

15. DIFFERENT VARIABLE REGIONS  DIFFERENT ANTIGEN-BINDING SITEs  DIFFERENT SPECIFICITies
the same applies to TCRs!

16. ANTIGEN BINDING FRAGMENT (Fab) CONTAINS HYPERVARIABLE REGIONS
DNA recombination of gene segments encoding these regions (variable heavy and light polypeptide chains) gives a huge number of variability during B cell development in the bone marrow.
Aka. Somatic recombination

17. COMPLEMENTARY DETERMINING REGIONS (CDR)
Epitope
CDR1
CDR2
CDR3
Light chain
Heavy chain

18. Isotype VARIABILITY IN DIFFERENT REGIONS OF THE Ig
DETERMINES Ig SPECIFICITY OR CLASS
Isotype
Sequence variability of H/L-chain constant regions
Sequence variability of H/L-chain constant regions

19. !
Human Immunoglobulin Classes encoded by different structural gene segments (isotypes)
Heavy chain types:
IgG - gamma (γ) heavy chains
IgM - mu (μ) heavy chains
IgA - alpha (α) heavy chains
IgD - delta (δ) heavy chains
IgE - epsilon (ε) heavy chains
Light chain types:
kappa (κ)
lambda (λ)

20. ISOTYPE SWITCHING
Isotype Switching during B Cell Development During the initial stages of a B cell’s primary response to antigen, it produces and secretes IgM. Later in the primary response or during subsequent responses, different heavy chain isotypes may be expressed by the progeny of the original IgM-producing clone. Such “switching” occurs at the DNA level, resulting in the production of an Ig protein with the same V region but a different C region. Thus, over the lifetime of a B cell clone, it may produce antibodies of the same specificity but different isotypes.

21. PHASES OF B CELL RESPONSE!
PHASES OF B CELL RESPONSE
Gene rearrangement in an immature B cell leads to the expression of membrane-bound IgM and IgD on the mature B cell surface. After encounter with an antigen this isotype is produced in a secreted form by the plasma cell (IgM largely while IgD only is small amounts). All isotypes (IgA, IgD, IgE, IgG and IgM) can be made in two forms: the bounded immunoglobulins serving as BCRs, and soluble secreted antibodies by the plasma cells. (The difference b/w membrane bound and secreted is a hydrophobic part and hydrophilic parts at the carboxy terminus, respectively).

22. MAIN CHARACTERISTICS OF ANTIBODY ISOTYPES

23. MAIN CHARACTERISTICS OF ANTIBODY ISOTYPES

24. ANTIBODY PRODUCTION DURING THE
PRIMARY AND THE SECONDARY IMMUNE RESPONSES
Level of antibodies
secondary response against antigen A
Primary response against antigen A
primary response against antigen B
Days
napok
Antigen A
Antigen A and B

25. ! ANTIBODY PRODUCTION DURING THE
PRIMARY AND THE SECONDARY IMMUNE RESPONSE

26. ! ! EFFECTOR FUNCTIONS OF ANTIBODIES NEUTRALIZATION OPSONIZATION ADCC
Antibody-mediated immune responses
NEUTRALIZATION
OPSONIZATION
ADCC
MAST CELL DEGRANULATION
COMPLEMENT FIXATION

27. Covering of the pathogen’s surface prevents replication and growth
NEUTRALIZATION
Covering of the pathogen’s surface prevents replication and growth

28. Complement binding site Binding to Fc receptors
Antigen binding
Complement binding site
Placental transfer
Binding to Fc receptors

29. OPSONIZATION Flagging a pathogen
Antigen binding portion (Fab) binds the pathogen, the Fc region binds phagocytic cells Fc-receptors speeding up the process of phagocytosis

30. Antibody Dependent Cellular Cytotoxicity
(ADCC)

31. MAST CELL DEGRANULATION
FcεRI +
IgEs
Mast cells, basophils and activated eosinophils in mucosal surfaces play a role in the defense against parasites and express the FcεRI.
FcεRI has such high affinity that the IgEs, specific for many different antigens- cannot dissociate.
Upon antigen binding and FcεRI cross-linking the mast cell is activated (degranulation) inflammatory mediators released, acting on vessel permeability- swelling, pain etc, and acting on smooth muscle cells  Sneezing, coughing, vomiting, diarrhea.
Directly killing the parasite by toxic granule content or Indirectly flushing it.
An unnecessary response to an innocuous substance (pollens etc.) are an unfortunate side effect of the highly specialized and powerful antibodies.
(A) High-affinity FcRs on the surface of the cell bind monomeric Ig before it binds to antigen. (mast cell)
(B) Low-affinity FcRs bind multiple Igs that have already bound to a multivalent antigen. (macrophage, NK cell)

32. Complement binding site Binding to Fc receptors
Antigen binding
Complement binding site
Placental transfer
Binding to Fc receptors

33. IgM and IgGs activate the classical pathway of the complement system
COMPLEMENT FIXATION
IgM and IgGs activate the classical pathway of the complement system

34. Bacterium Macrophage C3b Complement receptor OPSONIZATION BY C3b
Both antibodies and complement proteins are opsonins.
C3b
Complement receptor
Macrophage

35. IMMUNOGLOBULIN ISOTYPES HAVE EACH THEIR SPECIFIC CAPABILITIES

36. Complement binding site Binding to Fc receptors
Antigen binding
Complement binding site
Placental transfer
Binding to Fc receptors
FcRn on the placenta facilitate the transfer of maternal IgG to the fetus’s circulation

37. PRODUCTION OF IMMUNOGLOBULINS
IgG transport is so efficient that at birth babies have as high a level of IgG in their plasma as their mothers
These transfers are a form of passive immunization. The babies protection by IgG and IgA is against those pathogen that the mother has mounted
At the first year (esp.3-12m) maternal IgGs are catabolized and breast feeding diminishes so babies become most susceptible/vulnerable to infections

38. Pathological consequences of placental transport of IgG
(hemolytic disease of the newborn)
Passive anti-D IgG
anti-Rh
IgM
Rhesus incompatibility: In case a fetus is Rh+ (meaning he expresses the D antigen on his RBCs surface) and the mother is RH- (no D antigens and no anti-D antibodies) after the first delivery when some fetal RBCs mix with maternal circulation, the mother will initiate a primary immune response towards the D antigen. These antibodies as it is the first immune response will be of IgM isotype and therefore not able to pass the placenta. However, with time, isotype switching might take place that will result in the production of IgG antibodies against the D antigen. These are now able to pass through the placenta, thus, in the second pregnancy if the fetus is Rh+ his RBCs will be attacked by maternal anti-D IgG antibodies, causing the mild to severe ‘hemolytic disease of the newborn’ (destruction of red RBCs anemia)
To avoid that, i.m. anti-D IgGs are administered to the mother. These will bind any D antigens on fetal RBCs that entered the mothers circulation and prevent her from developing anti-D antibodies. The IgGs eliminate the RBCs coated with D antigens through opsonisation and elimination by macrophages and granulocytes.
… One might say, well IgGs pass the placenta, then these anti D antibodies can attack the fetus RBCs- true, but the clinical course of such an event is benign and requires no treatment.

39. DIMERIC IgA
(Transcytosis)
IgA dimers are in the highly vulnerable mucosal epithelia lining the GI, respiratory, urinary and genital tracts, the eyes, nose and throat