The document discusses humoral immunity and the complement system. It defines key terms and describes the three stages of humoral immunity: B cell maturation in the bone marrow, activation of B cells by antigens, and differentiation of activated B cells into plasma cells and memory B cells. It also explains the classical, alternative, and lectin pathways of complement system activation and how they converge in the formation of the membrane attack complex. Finally, it outlines the biological activities and regulation of the complement system.

1. HUMORAL IMMUNITY and
COMPLEMENT SYSTEM
EFFORTS BY – HIMANSHI GUPTA

2. Basic terms
 Immunology - study of structure and function of the immune system
 Immunity - resistance of a host to pathogens and their toxic effects
 Immune system - cells, tissues, and molecules that mediate resistance to
infections
 Immune response - collective and coordinated response to the introduction of
foreign substances in an individual mediated by the cells and molecules of the
immune system

3. Types of immunity
1. Innate (non-adaptive)
 first line of immune response
 relies on mechanisms that exist before infection
2. Acquired (adaptive)
 Second line of response (if innate fails)
 relies on mechanisms that adapt after infection
 handled by T- and B- lymphocytes
 one cell determines one antigenic determinant

4. Adaptive immunity: mechanisms
 Cell-mediated immune response (CMIR)
 T-lymphocytes
 eliminate intracellular microbes that survive within phagocytes or other infected cells
 Humoral immune response (HIR)
 B-lymphocytes
 mediated by antibodies
 eliminate extra-cellular
microbes and their toxins

5. Humoral immunity
 The humoral immunity is mediated by antibody molecules that are secreted by
plasma cells.
 Antibodies functions as effector of the humoral response by binding to antigen
and facilitating its elimination.
 It can be divided into three broad stages :
1) generation of mature, immunocompetent B cells k/a maturation
2) activation of B cells upon interaction with antigen
3) differentiation of activated B cells into plasma cells and memory B cells

7. B cell development

8. B cell maturation

9.  B cell development begins as lymphoid precursor cells differentiate into B-lineage
cell – the progenitor B cell (pro-B cell) .
 Precursor Bcells (pre B cells) requires the microenvironment provided by the bone
marrow stromal cells.
 The stromal cells plays two important roles : they interact directly with pro B and
pre B cells ; and they secrete various cytokines mainly IL-7 that support the
developmental process.
 The interaction b/w receptor c-kit and surface molecule SCF activates the tyrosine
kinase activity of c-Kit , and the pro B cell begins to divide and differentiate into a
pre B cell.
 Then IL-7 derives maturation process.

10. Ig gene arrangement produces immature B cells

11. Self reactive B cells are selected
against in bone marrow
 Only about 10 % cells are recruited into the circulating B cell pool , rest 90% cells
die each day without ever leaving the bone marrow.
 Some of this loss is attributable to negative selection and subsequent elimination
of immature B cells that express auto-antibodies against self antigens in the bone
marrow.
 Later Nemazee and Burki showed that negative selection does not always result in
deletion . Instead , maturation of the self reactive cells is arrested while the B cell
‘edits’ the light chain gene of its receptor.
 As a result, cells will begin to express an ‘edited’ mIgM with a different light
chain and can escape negative selection.

12. B cell activation and proliferation
Thymus independent and Thymus dependent
antigen

13. Mechanism
TI-1 antigens
 LPS acts as TI-1 antigen
 LPS interacts with two different receptors on B cell . One is Toll – like receptor TLR4 and
other is B cell receptor BCR .
 Only a B cell population bears BCRs specific for LPS , but all of them have TLR4. Those B
cells with BCRs that recognize LPS are stimulated to divide and secrete anti – LPS antibody ;
and other B cells are induced to divide and differentiate into antibody – secreting cells by
the interaction of LPS with TLR4.
 Nude mice experiment demonstrates that TI-1 antigens are truly T cell independent .

14. TI-2 antigens
 They are highly repetitious molecules such as polymeric proteins or bacterial cell
wall polysaccharides with repeating units.
 Activates B cells by extensively cross linking the mIg receptor.
 Points of differences
i) mitogens
ii) require cytokines derived from TH cells .
iii) activate mature B cells and inactivate immature B cells

15. Two types of signal drive B cells into and through
the cell cycle
• Naïve B cells are non dividing in G0
stage of cell cycle.
• Activation drives these cells to G1 to
S phase , requires two sets of signals
: signal 1 and signal 2
• Once the B cell has been stimulated
by the signals , results in full
activation, leading to cell division
and differentiation into memory cells
and plasma cells.

16. Ig-α and Ig-β heterodimers
 All isotypes of mIg have very short cytoplasmic tails .
 mIgM & mIgD on B cell extend into cytoplasm by only 3 amino acids
 mIg A – 14aa
 mIg G & mIgE -28aa
 Discovery – mIg is associated with the disulfide – linked heterodimer Ig-α/Ig-β , forming the
BCR. Thus BCR is functionally divided into ligand binding Ig molecule and the signal
transducing Ig-α/Ig-β heterodimer.
 Ig-α chain has long cytoplasmic tail – 61aa & Ig-β has 48aa . They also contain 18 –residue
motif termed as immunoreceptor tyrosine –based activation motif (ITAM) .
 Interactions with cytoplasmic tails of heterodimers transduce the stimulus produced by
cross – linking of mIg molecules into an effective intracellular signal.

17. B cell receptor

18. B cell signalling is initiated by antigen binding and
induces many signal transduction pathways

19. PKC- protein kinase C
GEF – guanine nucleotide exchange
factor
IP3 – inositol 1,4,5 – triphosphate
DAG – diacyl glycerol
PLCγ2 – phospholipase Cγ2
Btk – bruton’s tyrosine kinase
BLNK – B cell linker protein
Protein tyrosine kinases – src kinase,
Lyn, Blk, Fyn

20. The humoral response
• Depends on naïve lymphocytes and memory
lymphocytes .
• Primary response characterised by production of
plasma cells and memory B cells.
• Antibody concentration in serum depends on nature
of antigen , the route of antigen administration ,
species or strain being immunized and the presence
or absence of adjuvants.
• During lag phase , naïve B cells undergo clonal
selection , expansion and differentiation.
• Duration of lag phase varies with the nature of
antigen.
• Greater magnitude of secondary response – the
population of memory cells specific for a given
antigen is considerably larger than the population of
naïve cells .
• Affinity maturation and class switching are also
responsible.

22. THE COMPLEMENT SYSTEM
 Paul Ehrlich coined the term ‘complement’ , defining it as “ the activity of blood serum
that completes the action of antibody” .
 Lytic action of complement is a result of interactions of a complex group of proteins.
 Plays a key role in both innate and adaptive immunity .
 Complement is a system of plasma proteins that can be activated directly by
pathogens or indirectly by pathogen-bound antibody, leading to a cascade of
reactions that occurs on the surface of pathogens and generates active components
with various effector functions.
 Complement has 3 pathways : the alternative , classical and lectin pathways and the
final step leads to the formation of membrane – attack complex (MAC) .

23. Classical pathway

25. Alternative pathway
• Antibody independent pathway , therefore the
component of innate immune system.
• Initiated by cell surface constituents that are
foreign to host , like bacterial cell walls
• C3 contains an unstable thioester bond ,
subjected to slow spontaneous hydrolysis to
yield C3a and C3b .
• The active C3bBb generated can activate
unhydrolyzed C3 to produce more C3b and
forms active C3bBbBb.

26. Lectin pathway
 Originates when host protein binds microbial surfaces .
 It activates complement which binds to mannose
residues .
 Antibody independent but uses proteins of
classical pathway .
 MASP-1 and MASP-2 binds to MBL , causes cleavage
and activation of C4 and C2 and finally forms
C5 convertase .

27. Pathways converge at MAC
• This complex forms a large channel
through membrane of the target
cell , enabling transport of small
molecules and ions .
• The C5b component becomes
inactive until stabilised by binding
of C6.
• Upto this point , all reactions takes
place on the hydrophilic surfaces of
membranes but as C5b6 binds to C7
, complex undergoes structural
transition that exposes hydrophobic
regions .
• Final step is polymerization of C9 , a
perforin –like molecule and its
binding to C5b678 complex .

28. Regulation of complement system

29. Biological activities of complement
activation
OPSONISATION
 In cells such as neutrophils , which performs phagocytosis , opsonins coat bacteria and
facilitate their removal .
 C3b accounts for most of the complement opsonic activity .
CELL LYSIS
 Complete complement activation leads to cell lysis .
 Typical targets include bacteria and enveloped viruses .
 Host erythrocytes , platelets and lymphocytes may also become victims.

30. CELL RECRUITMENT AND ACTIVATION
 Role in clinical syndrome anaphylaxis .
 Low – molecular weight fragments C4a, C3a and C5a are known as anaphylatoxins .
 They activate mast cells and basophils .
IMMUNE CLEARANCE
 Immune complexes of antibody – antigen continuously forming in circulation which
infections .
 They can become insoluble and fixed in tissues .
 Covalent binding of C3b to antibody in a complex inhibits lattice formation and maintains
solubility , deposits them in liver and spleen where they are released and taken up by
macrophages.

31. Thank you