ANTIBODIES

Antibodies are protein molecules called immunoglobulins which are produced in

response to the entry of foreign antigens into the body, via infection. Several kinds
of antibodies may be formed against a single antigen possessing several epitopes.
Antibodies are released from plasma cells. Antibodies are found in many body
fluids such as milk, tears, urine and in secretions of the respiratory, digestive and
genitourinary tracts.
Structure of antibody
Antibody is a Y –shaped four polypeptide chain molecule (Fig. 4.1). Amongst the
four chains, two are light (L) chain and other heavy (H) chains. Both heavy and
light chains are joined together by disulphide bonds. Both chains vary in their
molecular weights; ‘L’ has a molecular weight of 25,000 Daltons while the ‘H’ has
50,000 Daltons. H chains are used to differentiate each class of antibody, as chains
are structurally and antigenically different. Each different heavy chains in each
class are designated with Greek letter α (alpha), δ (delta), γ (gamma), μ(mu), ε
(epsilon). In ‘L’ chain, k (kappa) and λ (lambda) are found. However, molecules
can contain either k or λ chain but never both together. When antibody is treated
with proteolytic enzyme papain in the presence of cysteine, it cleaves it into three
fragments. Two identical fragments possess the antigen-binding sites or Fab. The
remaining fragment lacks the ability to bind antigen, so it is named fragment
cystalizable (Fc).
Various functions of Fc fragment are
(1) Binds complement leads to complement fixation;
2) Binds to cell receptors (3) determine passage across the placental barrier.
 fig 4.1

TYPES OF IMMUNOGLOBULINS OR ANTIBODIES

IgG
Immunoglobulin G is the most abundant immunoglobulin in the body. 2. It
constitutes about 75%of the total Igs. 3. It is found in both intravascular and
extravascular pools. 4. During primary response, low levels of IgG are produced
but in the secondary response, it is produce in high level. 5. It is a glycoprotein
with molecular weight of 150,000 Daltons. 6. IgG present in normal serum is about
12 mg/ml. 7. IgG is the only class of Igs that can cross the placenta and responsible
for infant protection. Breast milk also contains IgG along with IgA.
IgM
1.IgM is the largest antibody; it tends to remain in the blood and is efficient in
killing bacteria. 2. IgM is a glycoprotein with molecular weight of about 900,000
Daltons. 3. Normal serum contains about 10% of IgM and its half life is about 5
days. 4. Its structure consists of pentamer with five immunoglobulins. 5. IgM is
found in serum but is completely absent in body cavities or secretions. 6. During
immune response, IgM is the first antibody to appear. 7. It is the first antibody to
be synthesized by fetus by about 20 weeks of age. 8. IgM is usually short lived;
hence their presence in the serum indicates recent infection.
IgA
1. Human IgA constitutes about 13% (2.1mg/ml) of the antibody in human
serum. 2. The IgA is present in secretions (tears, saliva, nasal secretions,
bronchial and digestive tract mucus and mammary gland secretions) 3.
Serum IgA has molecular weight of 160,000 Daltons. 4. There are two
classes of IgA: IgA 1 and IgA 2.
IgE
1IgE is found in trace amounts in the blood. 2. It can triggers allergies. 3. Human
IgE make up less than 0.003% (0.4µg/ml) of the antibody in serum. 4. It has
molecular weight 190,000 Daltons and half life is about two days. 5. It is heat
labile and gets inactivated by heating it at 560 C for 30 minutes.
6. It is produced in the linings of the respiratory and intestinal tracts. 7. Increased
levels of IgE are seen in type I hypersensitivity.
IgD
1. IgD remains membrane bound and regulates the cell’s activation. 2. Its
molecular weight is approximately 175,000 Daltons. 3. It is present on the
surface of B lymphocytes. 4. IgD constitutes less than 1% (40µg/ml) of the
antibody in human serum.
COMPLEMENT SYSTEM
Complement system is an enzyme system present in the serum. This system is
activated by antigen and antibody interaction. Mostly, complemenst present in the
body are found in inactive form. Antigen and antibody reaction induces the activity
of complement. Activation of complement occurs by two pathways
1. Classic pathway – it is set of events, during this components of complement
react in sequence which results in immune cytolysis.
2. Alternate pathway
Complement takes part in various functions such as
1. Mediates immunological membrane damage causing bacteriolysis or cytolysis.
2. Involved in hypersensitivity reactions
3. It has antiviral activity.
4. Promotes phagocytosis.
5. Amplifies the inflammatory response.

IMMUNE RESPONSE
The specific reactivity induced by an antigenic stimulus in a host is called as
immune response. There are two types of immune response such as
1. Antibody mediated response (humoral)
2. Cell mediated response
Antibody mediated (humoral) response
This kind of response gives protection against bacterial pathogens and viruses that
invade through respiratory or gastrointestinal tract. It prevents recurrence of viral
infections and participates in the pathogenesis of immediate hypersensitivity
reactions. Humoral responses occur in two ways
1. Primary response
2. Secondary response
Primary response
When an antibody has initial contact with a new antigen (primary stimulus), it
produces the primary response. This response is slow to develop and is short lived.
The antibody titre is low and it does not persist for long time. IgM is predominant
during primary response.
Secondary response
This is the response to subsequent stimuli with the same antigen. It lasts for a long
period and is effective. The antibody titre is high and persists for a long time. IgG
is predominant during secondary response.

Cell mediated immune response (CMI)

Cell mediated immune response is a specific immune response. This response is
mediated by T lymphocytes. This immunity provides resistance against fungi,
viruses and intracellular bacterial pathogens. It involves in graft versus host
reaction and provides immunity against cancer. CMI is provided by T lymphocytes
and their products (lymphokines). Monokines are secreted by monocytes and
macrophages. Combination of lymphokines and monokines constitute cytokines.
Cytokines
These are soluble products secreted by lymphocytes, macrophages, neutrophils and
natural killer cells. Cytokines are classified into two kinds
Afferent cytokines
It includes interleukins, interferons and B cell differentiating factor. They are
helpful in induction of immune response.
Efferent cytokines
It includes macrophage inhibiting factor, leucocyte inhibiting factor and
lymphotoxins. These molecules involved in suppression of immune response.
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