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1. Immunoglobulins:
Structure and
Functions
BY: RAISA SAIF

2. Introduction
 Immunoglobulins, also known as
antibodies, are glycoprotein molecules
produced by plasma cells in response
to an antigen.
 They play a crucial role in the immune
system's defense against pathogens.

3. Antibodies are the antigen binding proteins present
on the B-cell membrane and secreted by plasma
cells.
Secreted antibodies circulate in the blood, where
they serve as the effectors of humoral immunity by
searching out and neutralizing antigens or marking
them for elimination.
All antibodies share structural features, bind to
antigen, and participate in a limited number of
effector functions.

4. Structure of Immunoglobulins
 Immunoglobulins have a Y-shaped
structure composed of four
polypeptide chains: two heavy chains
(H) and two light chains (L).
 The chains are connected by disulfide
bonds and hinge regions, allowing
flexibility and binding versatility.

6. Classes and Isotypes
 There are five classes of
immunoglobulins: IgG, IgM, IgA, IgD,
and IgE.
 Each class has distinct structural and
functional properties.
 Isotypes refer to the different
variations within each class.

7. IgG
 IgG is the most abundant immunoglobulin in the
blood and tissues.
 IgG, the most abundant class in serum,
constitutes about 80% of the total serum
immunoglobulin.
 The IgG molecule consists of two heavy chains
and two or two light chains.
 There are four human IgG subclasses,
distinguished by differences in -chain sequence
and numbered according to their decreasing
average serum concentrations: IgG1, IgG2,
IgG3, and IgG4

8.  IgG1, IgG3, and IgG4 readily cross the placenta
and play an important role in protecting the
developing fetus.
 IgG1 and IgG3 bind with high affinity to Fc
receptors on phagocytic cells and thus mediate
opsonization. IgG4 has an intermediate affinity
for Fc receptors, and IgG2 has an extremely low
affinity
 It provides long-term protection against
infections.
 Functions include neutralizing toxins, activating
complement, and enhancing phagocytosis

11. IgM
 IgM is the first immunoglobulin produced during
an initial immune response.
 IgM accounts for 5%–10% of the total serum
immunoglobulin, with an average serum
concentration
 It exists as a pentamer, allowing efficient antigen
binding.
 Functions include agglutination, complement
activation, and opsonization.

12.  Fc regions in the center of the
pentamer and the ten antigen-binding
sites on the periphery of the molecule.
 IgM is the first immunoglobulin class
produced in a primary response to an
antigen, and it is also the first
immunoglobulin to be synthesized by
the neonate.

14. IgA
 IgA constitutes only 10%–15% of the total immunoglobulin in
serum, it is the predominant immunoglobulin class in external
secretions such as breast milk, saliva, tears, and mucus of
the bronchial, genitourinary, and digestive tracts.
 In serum, IgA exists primarily as a monomer, but polymeric
forms (dimers, trimers, and some tetramers)
 It plays a crucial role in mucosal immunity and prevents
pathogen attachment.
 Functions include neutralization, agglutination, and blocking
bacterial adhesion.

15.  Breast milk contains secretory IgA and
many other molecules that help
protect the newborn against infection
during the first month of life.
 Because the immune system of
infants is not fully functional, breast-
feeding plays an important role in
maintaining the health of newborns.

17. IgD
 IgD is primarily found on the surface of
B cells.
 Its exact function is not fully
understood, but it may be involved in
the activation of B cells.

19. IgE
 IgE is involved in allergic responses and defense
against parasitic infections.
 Its extremely low average serum concentration is (0.3
g/ml).
 IgE antibodies mediate the immediate hypersensitivity
reactions that are responsible for the symptoms of hay
fever, asthma, hives
 It triggers the release of inflammatory mediators from
mast cells and basophils.

22. Antigen Binding
 The variable region of the
immunoglobulin molecule contains
antigen-binding sites.
 Each immunoglobulin can bind to
specific antigens through these sites.
 The antigen-binding site is formed by the
variable regions of both the heavy and
light chains.

23. Constant Regions
 The constant region of
immunoglobulins determines their
class and effector functions.
 It interacts with various immune cells
and effector molecules to mediate
specific immune responses.

24. Antibody Diversity
 The immune system generates a vast
array of different immunoglobulins
through gene rearrangement.
 This process, known as V(D)J
recombination, creates diversity in the
antigen-binding sites.

25.  During the primary immune response,
IgM is produced first, followed by IgG.
 The secondary immune response
results in a faster and more robust
production of antibodies.
Primary and Secondary Immune
Responses

26. Opsonization
 Immunoglobulins can act as opsonins,
marking pathogens for phagocytosis
by immune cells.
 Opsonization enhances the efficiency
of phagocytosis and pathogen
clearance.

27. Neutralization
 Immunoglobulins can directly
neutralize pathogens and toxins by
binding to their surfaces.
 This prevents the pathogens/toxins
from interacting with host cells and
causing harm.

28. Agglutination
 Immunoglobulins can cause
pathogens to clump together, making
them easier targets for immune cells.
 Agglutination helps restrict the spread
of pathogens and enhances their
clearance.

29. Allergic Reactions
 IgE-mediated allergic reactions occur
when immunoglobulin IgE binds to
allergens.
 This triggers the release of histamine
and other inflammatory mediators,
causing symptoms like itching,
sneezing, and asthma.

30. Immunoglobulin Deficiencies
 Immunoglobulin deficiencies can lead
to increased susceptibility to
infections.
 Common deficiencies include
selective IgA deficiency and common
variable immunodeficiency (CVID).

31. Conclusion
 Immunoglobulins are essential
components of the immune system.
 Their diverse structures and functions
contribute to effective immune
responses against pathogens.