• Innate immunity:
• It is non specific immunity
• It is the natural resistance with which a person is born.
• It provides resistance through following barriers:
1. Anatomic barriers (Skin,mucous membranes)
2. Physological barriers (temperature, pH)
3. Phagocytic Barriers (cells that eat invaders)
• Adaptive (or acquired) immunity :
• It is specific immunity
• It occurs after an exposure to an antigen.
• Creates immunological memory after an initial response to a
specific pathogen, leading to an enhanced response to
subsequent encounters with that same pathogen.
Active and Passive Immunity
• Passive immunity: Transfer of Ab with a given
• Passive immunity is the transfer of active
immunity, in the form of readymade antibodies,
from one individual to another
• Important for
Mother to new born
• Active immunity: Production of Ab in host when
it comes in direct contact with Ag.
Cells of the immune system
Lymphocytes constitute 20%–40% of the body’s white
blood cells and 99% of the cells in the lymph.
There are approximately 1011 (range depending on
body size and age: ~1010–1012) lymphocytes in the
These lymphocytes continually circulate in the blood
and lymph and are capable of migrating into the
tissue spaces and lymphoid organs.
The lymphocytes can be broadly subdivided into
T cells, and
natural killer cells
• Lymphocytes proliferate and eventually
• Memory cells
• Effector cells
1. MEMORY CELLS:
• Some of the progeny of B and T
lymphoblasts differentiate into memory
• Memory cells look like small lymphocytes
but can be distinguished from naive cells by
the presence or absence of certain cell
2. EFFECTOR CELLS
• Effector cells function in various ways to
eliminate antigen. (have short life span)
• Effector cells of the B cell are called the
Plasma cells—the antibody-secreting
• The effector cells of the T-cell include
– T helper cell (TH cell)
– T cytotoxic cell (TC cell).
• Mature in Bursa of Fabricus/bone marrow.
• Mature B cells are distinguished from other
lymphocytes as they have membrane-bound
antibody molecules which serve as receptors
for antigen. They are known as B-cell antigen
• Interaction between antigen and the
membrane-bound antibody on a mature B cell,
as well as interactions with T cells and
macrophages, causes activation of B cells.
• In this process, the B cell divides repeatedly and
differentiates over a 4- to 5-day period,
generating a population of plasma cells
(effector cells of B lymphocytes) and memory
• They also act as antigen-presenting cells.
• The Major Histocompatibility Complex (MHC) is a set of
molecules displayed on cell surfaces that are
responsible for lymphocyte recognition and "antigen
• MHC class II normally occurs only on antigen-presenting
• MHC class I occurs on all nucleated cells
Maturation in thymus.
Like B lymphocytes, these cells have membrane receptors for
antigen named as T-cell receptor (TCR) .
TCR recognizes only antigen that is bound to particular classes of
antigen-presenting cells or on virus-infected cells, cancer
Most T cells recognize antigen only when it is bound to the
major histocompatibility complex (MHC).
All T-cell receptors can be distinguished by the presence of
one or the other of two membrane molecules, CD4 and
T cells that express the membrane
glycoprotein molecule CD4 are restricted to
recognizing antigen bound to class II MHC
T cells expressing CD8, a dimeric membrane
glycoprotein, are restricted to recognition of
antigen bound to class I MHC molecules.
CD4 T cells generally function as T helper (TH)
cells and are class-II restricted;
CD8 T cells generally function as T cytotoxic
(TC) cells and are class-I restricted.
TH cells are activated by recognition of an antigen–class
II MHC complex on an antigen-presenting cell.
After activation, the TH cell begins to divide and gives
rise to effector cells.
These TH cells secrete various cytokines, which play a
central role in the activation of B cells and T cells.
A cytotoxic T cell is a T lymphocyte that
kills cancer cells, cells that are infected (particularly with
viruses), or cells that are damaged in other ways.
• Cell-mediated immunity is an immune
response that does not involve antibodies but
rather involves the activation of phagocytes,
natural killer cells (NK), cytotoxic T-
lymphocytes, and the release of
various cytokines in response to an antigen by
• The humoral immunity is the aspect
of immunity that is mediated by
secreted antibodies produced in the cells of the
• B Cells transform into plasma cells which secrete
antibodies. Secreted antibodies bind to antigens on
the surfaces of invading microbes (such as viruses
or bacteria), which marks them for destruction.
Overview of Humoral Immunity
• Ingestion (phagocytosis) of foreign matter by special blood
cells called macrophages.
• The macrophages digest the infectious agent and then display
some of its components on their surfaces.
• Cells called helper-T cells recognize this presentation.
• Activated helper-T cells use chemical signals to contact B-cells,
which then begin to multiply rapidly as well.
• B-cell descendants become either plasma cells or B memory
• The plasma cells begin to manufacture huge quantities of
antibodies that will bind to the foreign invader (the antigen)
and prime it for destruction. B memory cells retain a
"memory" of the specific antigen that can be used to mobilize
the immune system faster if the body encounters the antigen
later in life.
Overview of cell mediated
• The cell-mediated immune response involves cytotoxic T-
cells, or killer-T cells.
• Body cells that have been infected by foreign matter often
present components of that material on their surfaces.
• Killer-T cells recognize these displays and respond by ingesting
or otherwise destroying the infected cell. Killer-T cells are also
important in the body's defenses against parasites, fungi,
protozoans, and other larger cells that might have found their
way into the body. The killer-T cells recognize these large
invaders by their foreign proteins and then destroy them.
Natural killer cells (NK cells) are large, granular
lymphocytes that do not express the set of
surface markers typical of B and T cells .
Display cytotoxic activity against a wide range of
Constitute 5%–10% of lymphocytes in human
IgG is an antitumor and antiviral antibody that
binds to the surface of tumor cells.
Because NK cells express CD16, a membrane
receptor for Ig G antibody, they can attach to these
antibodies and subsequently destroy the targeted
The mononuclear phagocytic system consists of
monocytes circulating in the blood and
macrophages in the tissues.
Monocytes circulate in the bloodstream for
about 8 h, during which they enlarge; they then
migrate into the tissues and differentiate into
specific tissue macrophages or, into dendritic
They also act as antigen-presenting cells.
• Macrophages are capable of ingesting and digesting
exogenous antigens, such as whole microorganisms and
insoluble particles, and endogenous matter, such as
injured or dead host cells and cellular debris
• In the first step in phagocytosis, macrophages are
attracted by and move toward a variety of substances
generated in an immune response; this process is called
• The next step in phagocytosis is adherence of the
antigen to the macrophage cell membrane. Adherence
induces membrane protrusions, called pseudopodia, to
extend around the attached material.
• Fusion of the pseudopodia encloses the material
within a membrane-bounded structure called a
phagosome, which then enters the endocytic
• In this pathway, a phagosome moves toward the
cell interior, where it fuses with a lysosome to form
a phagolysosome. Lysosomes contain lysozyme and
a variety of other hydrolytic enzymes that digest the
• The digested contents of the phagolysosome are
then eliminated in a process called exocytosis
The granulocytes are classified on the basis of cellular
morphology and cytoplasmic staining characteristics
The neutrophil has a multilobed nucleus and a granulated
cytoplasm that stains with both acid and basic dyes
The eosinophil has a bilobed nucleus and a granulated
cytoplasm that stains with the acid dye eosin red (hence its
The basophil has a lobed nucleus and heavily granulated
cytoplasm that stains with the basic dye methylene blue.
Both neutrophils and eosinophils are phagocytic, whereas
basophils are not.
Originate in the bone marrow.
They function as antigen-presenting cells
They capture bacteria, process them
intracellularly, and resulting protein fragments
are presented to T cells.
Mature dendritic cells express very high levels
of MHC Class II molecules which mediate the
presentation of antigen to T cells.
Organs of the Immune System
Primary Lymphoid Organs (central)
Sites for development and maturation of lymphocytes
Secondary Lymphoid Organs (peripheral)
Sites for the initiation of an immune response. Trap
antigens and provide microenvironment for
lymphocyte and Ag interaction.
Bone marrow is the flexible tissue found in the interior
In humans and mice, bone marrow is the site of B-cell
origin and development.
Immature B cells proliferate and differentiate here.
Stromal cells within the bone marrow secrete various
cytokines that are required for development.
The thymus is the site of T-cell development and
It is a flat, bilobed organ situated above the heart.
Each lobe is surrounded by a capsule and is divided into
lobules, which are separated from each other by strands of
connective tissue called trabeculae.
Each lobule is organized into two compartments:
the outer compartment, or cortex: densely packed with
immature T cells, called thymocytes
the inner compartment, or medulla: sparsely populated
As blood circulates under pressure, its fluid component (plasma)
seeps through the thin wall of the capillaries into the surrounding
Much of this fluid, called interstitial fluid, returns to the blood
through the capillary membranes.
The remainder of the interstitial fluid, now called lymph, flows
from the spaces in connective tissue into a network of tiny open
lymphatic capillaries and then into a series of progressively larger
collecting vessels called lymphatic vessels.
In this way, the lymphatic system captures fluid lost from the blood
and returns it to the blood, thus ensuring steady-state levels of
fluid within the circulatory system.
When a foreign antigen gains entrance to the
tissues, it is picked up by the lymphatic system
and is carried to various organized lymphoid
tissues such as lymph nodes, which trap the
Lymph nodes are the sites where immune
responses are mounted to antigens in lymph.
They are encapsulated bean shaped structures
packed with lymphocytes, macrophages, and
As lymph percolates through a node, any
particulate antigen that is brought in with the lymph
will be trapped by the cellular network of
phagocytic cells and dendritic cells
As antigen is carried into a regional node by the
lymph, it is trapped, processed, and presented
together with class II MHC molecules by dendritic
cells, resulting in the activation of TH cells that
activate B cells.
• The spleen plays a major role in mounting
immune responses to antigens in the blood
• It is a large, ovoid secondary lymphoid organ
situated high in the left abdominal cavity.
• While lymph nodes are specialized for
trapping antigen from local tissues, the
spleen specializes in filtering blood and
trapping blood-borne antigens.
An antibody (Ab), also known as an
immunoglobulin (Ig), is a large Y-shaped
protein produced by B-cells that is used by the
immune system to identify and neutralize
foreign objects such as bacteria and viruses.
Antibodies are heavy (~150 kDa) globular
Antibodies are secreted by a type of white
blood cell called a plasma cell
The production of antibodies is the main
function of the humoral immune system
• 1. HEAVY AND LIGHT CHAINS :
• All immunoglobulins have a four chain structure as their basic
• They are composed of two identical small light chains (23kD)
and two identical large heavy chains (50-70kD).
The heavy and light chains and the two heavy chains are held
together by inter-chain disulfide bonds
– Heavy Chain:
– There are five types of mammalian Ig heavy chain denoted by the
Greek letters: α, δ, ε, γ, and μ.
– The type of heavy chain present defines the class of antibody;
these chains are found in IgA, IgD, IgE, IgG, and IgM antibodies,
– Light Chain:
– In mammals there are two types of immunoglobulin light chain,
which are called lambda (λ) and kappa (κ).
3.VARIABLE (V) AND CONSTANT (C) REGIONS
The heavy and light chain could be divided into two
regions based on variability in the amino acid
These are the:
1. Light Chain - VL (110 amino acids) and CL (110
2. Heavy Chain - VH (110 amino acids) and CH (330-
440 amino acids)
Most H chains consist of one variable (VH) and three
constant(CH) domains.(IgG and IgA have three CH
domains,whereas IgM and IgE have four.)
• The variable regions are responsible for antigen
binding ,whereas the constant regions are
responsible for various biologic functions e.g.
complement activation and binding to cell
• 4.Fab Region:
• The arms of the Y contain the sites that can bind two
antigens (in general, identical) and, therefore,
recognize specific foreign objects. This region of the
antibody is called the Fab (fragment, antigen
binding) region. It is composed of one constant and
one variable domain from each heavy and light chain
of the antibody
• 6. Fc Region:
• The base of the Y plays a role in modulating immune
cell activity. This region is called the Fc (Fragment,
crystallizable) region. and is composed of two heavy
chains that contribute two or three constant
domains depending on the class of the antibody.
• Thus, the Fc region ensures that each antibody
generates an appropriate immune response for a
given antigen, by binding to a specific class of Fc
receptors, and other immune molecules, such
as complement proteins.
CLASSES OF IMMUNOGLOBINS:
• The immunoglobulins can be divided into five
different classes, based on differences in the
amino acid sequences in the constant region
of the heavy chains.
• 1. IgG - Gamma heavy chains
• 2. IgM - Mu heavy chains
• 3. IgA - Alpha heavy chains
• 4. IgD - Delta heavy chains
• 5. IgE - Epsilon heavy chains