The adaptive immune system eliminates pathogens through specialized cells and processes. It has two main responses - humoral mediated by antibodies from B cells, and cell mediated by T lymphocytes. Adaptive immunity is specific, diverse and develops immunological memory allowing faster responses. It involves lymphocytes, antigen presenting cells, and effector cells working together precisely eliminate pathogens.

1. Adaptive Immune Response
Dr.V.Suganthi M.Sc., D.MLT.,Ph.D
Assistant Professor
Department of Biotechnology
Sri Sankara Arts and Science College
Enathur,Kanchipuram

2. The adaptive immune system, also referred as the
acquired immune system, is a subsystem of
the immune system that is composed of
specialized, systemic cells and processes that
eliminates pathogens by preventing their growth.
This process of acquired immunity is the basis of
vaccination

3. Adaptive immune responses are of two types
 Humoral immune response
 Cell mediated immune response

4. Humoral immune response
Humoral immune responses are mediated by the
antibodies which are produced by activated B cells.
Antibodies recognize the microbial antigen, neutralize
the infectivity, and target the microbes to other
effector system for degradation.
Humoral immunity is the major type of immune
response against extracellular microbes and toxins
because the secreted form of the antibody can easily
bind and eliminate the microbes and toxins.
 Antibodies may bind to the microbes to promote their
phagocytosis in order to eliminate the infection.

5. Cell mediated immune response
 This is also called cellular immunity, and is mediated by T
lymphocytes.
 Cell mediated immunity plays an important role against
intracellular microbes, viruses, and some intracellular bacteria.
 The cellular immunity promotes the destruction of microbes by
direct killing or phagocytosis of the infected cells.
 Immunity against a pathogen is usually induced by the exposure of
microbial antigen to the host and is called active immunity.
 Immunity can also be transferred by serum or lymphocyte from an
immunized individual to a diseased individual and is called passive
immunity.
 Passive immunization is a rapid way to transfer the immunity in
the absence of active immunity. Passive immunization against toxin
and venoms is a life saving treatment in many lethal conditions
(Tetanus toxoid, snake antivenom).

6. Concept of humoral immunity
 The first concept of humoral immunity was given by Emil
von Behring and Shibasabro Kitasato;
 they showed for the first time that serum transferred from
a recovered diphtheria patient protected the recipient from
active diphtheria infection.
 The active ingredients are called antitoxins because they
nullify the effect of toxins.
 They won the noble prize for their landmark discovery.
• Von Behring and Kitasato demonstrated that serum—the
liquid, noncellular component recovered from coagulated
blood—from animals previously immunized with
diphtheria could transfer the immune state to
unimmunized animals.

7. Concept of Cellular immunity
 In 1883, even before the discovery that a serum
component could transfer immunity, Elie Metchnikoff ,
another Nobel Prize winner, demonstrated that cells also
contribute to the immune state of an animal.
 He observed that certain white blood cells, which he
termed phagocytes, ingested (phagocytosed)
microorganisms and other foreign material
 Nothing that these phagocytic cells were more active in
animals that had been immunized, Metchnikoff
hypothesized that cells, rather than serum components,
were the major effectors of immunity.

8. Feature of adaptive immunity
Specificity
The adaptive immunity is specific to a particular
antigen, which means specific antibodies are
produced against a particular antigen.
The structures present over the antigen that stimulate
the production of antibodies are called antigenic
determinants or epitopes.
Minute differences exist among lymphocytes that
express membrane receptors which are able to
distinguish fine differences present on the epitopes.
The specificity of immune system leads to a huge
population of lymphocytes that are antigenically
specific and are called lymphocyte repertoire.

9. Diversity
The ability of lymphocyte recognize a wide variety
of antigens is called diversity.
 In fact lymphocyte that contains receptors for
different antigen contributes to a large population of
extremely diverse lymphocyte clones.
Memory
The ability of the immune system to remember the
antigens and respond again to the same upon
exposure is called immunological memory.
The immune response against the second exposure of
the same antigen or subsequent exposure is usually
rapid and larger than the primary immune response.

10. Immunologic memory
This immunological memory means that an enhanced adaptive
response is mounted upon a second and subsequent exposures to
a given pathogen, so that signs of clinical illness are mitigated or
prevented. In other words, immunity is achieved because the
body has effectively “adapted” its defenses and acquired the
ability to exclude a particular pathogen.
Constant supply of resting B and T cells is maintained
throughout the body, with each lymphocyte expressing its
complement of unique antigen receptor proteins.
When a pathogen antigen enters the body for the first time, a
process of clonal selection takes place in which only those
lymphocytes bearing receptors specific for that antigen are
triggered to respond.

11. The selected cells leave the resting state and multiply to
generate daughter cells all expressing the same antigen-specific
BCR or TCR.
The attack on the pathogen by this first round of effector cells
is called the primary immune response.
The second (or subsequent) time that the particular pathogen
enters the body, it is met by an expanded army of clonally
selected, antigen-specific memory cells that undergo much more
rapid differentiation into effector cells than occurred during the
first antigen encounter.
The result is a stronger and faster secondary immune response
that eliminates the pathogen before it can cause illness.
New populations of memory cells are also produced during the
secondary response, ensuring that the host maintains long-term or
even lifelong immunity to that pathogen.

12. Tolerance
 The body avoids lymphocyte attacks on molecules present in its
tissues. This avoidance is called tolerance, the fifth aspect in which
the adaptive response exhibits more refinement than the innate
response .
Tolerance is established in two broad stages, each of which involves
multiple mechanisms.
The first stage, called central tolerance, occurs during early
lymphocyte development. The mechanisms of central tolerance are
designed to eliminate clones that recognize self antigens, thus
establishing a lymphocyte repertoire that targets “non-self.”
In the second stage, called peripheral tolerance, any B and T
lymphocytes that recognize self but somehow escaped the screening
of central tolerance and completed their development are
functionally silenced by another set of inactivating mechanisms

13. Cells of the adaptive immune system
Cells of the adaptive immune system includes
following.
• Lymphocytes
• Antigen presenting cells
• Effector cells

14. Lymphocytes
 Lymphocytes are the cells that mediate adaptive immunity.
 In the resting state they are inactive .na€ıve. precursor cells that
 need to be stimulated by antigen, and usually by other signals
before they become fully activated.
Two major classes of lymphocytes,
 As noted earlier, these comprise the T cells – which are in fact of
two main types, CD4 Tcells and CD8 Tcells – and the B cells.
 These are the .conventional. types of lymphocyte that play central
roles in adaptive immunity. Conventional T cells are also called αβ
T cells. There are in addition other types of
 less conventional (or unconventional.) lymphocytes, including
γδT cells and NKT cells

15. B Lymphocytes
 B lymphocytes are also small, resting cells, morphologically
indistinguishable from T cells.
 When they are appropriately activated their primary function is to
develop into plasma cells, which can be regarded as antibody-
synthesizing factories, or to become memory cells.
 In many cases B cells need help from T cells to become activated and
to develop into plasma cells, and the Tcells also control the type of
antibody that they make.
 This type of response, which is typically made in response to protein
antigens by B cells in specialized sites of secondary lymphoid tissues
(termed follicles), is therefore termed a T-dependent (TD) response.
 Another subset of B cells, located in a specialized site of the spleen
(called the
 marginal zone) can produce antibodies to other types of antigen, such
as polymeric carbohydrates, without needing any help from Tcells and
this is an example of a T-independent (TI) response.

17. T cells represent about 70% of the total lymphocyte population –
this varies depending on the activity of the immune response and
can be up to 90%
all T cells express CD3 on their surfaces, along with T cell
receptors (TCRs) which recognise specific antigens presented in
an MHC I or MHC II molecule
there are numerous different T cell subtypes with different roles,
which each have their own identifiable surface markers
T Lymphocytes

18. Types of Tcells
 helper T cells (CD4)
facilitate the activation of the immune response and
stimulate division and differentiation of various effector
cells
 cytotoxic T cells (CD8)
also known as killer or effector T cells – provide cell-
mediated immunity by targeting and killing infected
cells
 regulatory T cells (CD25 + FOXP3) –
also known as suppressor T cells – play a vital role in
limiting the immune response to prevent excessive
damage to tissues and organs
 memory T cells (CD62 + CCR7)
“remember” what has happened to allow the immune
system to mount a faster, more effective response should
the offending organism be foolish enough to return

19. CD8 T cells
 CD8 T cells represent the other main type of conventional Tcell.
 These Tcells possess multiple copies of a molecule called CD8 that is involved
in recognition of the cells with which they interact.
 Following activation, these T cells can become cytotoxic T cells capable of
inducing apoptosis in cells they recognize.
 CD8 T cells, like CD4 T cells, circulate in the bloodstream and migrate through
secondary lymphoid tissues.
 To function they also need to be activated and this occurs in the secondary
lymphoid tissues.
 When CD8 T cells are fully activated, they become cells with potent cytotoxic
activity.
 These cells leave the secondary lymphoid tissues, and enter peripheral sites of
inflammation and infection.
 Here they can kill virally infected cells.
They do so in two main ways –
 Through perforin and granzymes, and through Fas ligand– Fas interactions.
 As cytotoxic cells, CD8 T cells are a central component of adaptive immunity to
viruses such as the influenza virus.

20. • They can also produce cytokines that
• (i) are directly toxic, such as tumour necrosis
factor (TNF)-a, which induces apoptosis by
binding to death-inducing receptors on other cells
• (ii) can modulate or enhance the functions of
innate cells (e.g. IFN-c, the major macrophage-
activating cytokine), thus providing additional
mechanisms by which they may help to eliminate
infectious agents.

21. CD4 Helper and Regulatory T Cells
 CD4 T cells, which when activated become conventional helper T (Th)
cells or regulatory T (Treg) cells, are one of the two main types of T cells
 They are so-called because they possess a molecule called CD4 (in
multiple copies, of course) that is involved in recognition of the cells with
which they. interact.
 CD4 Tcells circulate in the bloodstream and migrate through secondary
lymphoid tissues. They are small, resting cells. To function, they need to
be activated and in primary responses this occurs in the secondary
lymphoid tissues
 There are at least four main ways in which CD4 Tcells can be instructed
to function, and the respective Tcell subsets are termed Th1, Th2, Th17
and Treg cells.
 These subsets stimulate different types of adaptive immune response,
either by
 recruiting or modifying the functions of the cells of innate immunity,
initiating responses in other adaptive cells or by bringing in new cells and
molecules into the response.

23. Th1 cells
Stimulate anti-microbicidal and cytotoxic effector
functions of immunity
 They also instruct B cells to develop into plasma
cells and secrete certain types of Antibodies that
can interact with some of these cells.
Th1 responses may be particularly important for
later defence against some types of bacterial and
viral infections (e.g. tuberculosis, influenza).

24. Th2 cells
• stimulate the barrier functions of immunity
• This type of response may be particularly
important for host defence or
• resistance against parasitic infections such as
worm infestations

25. Th17 cells
 are particularly efficient at recruiting neutrophils to
the site of infection;
 This type of response may be particularly important
for defence against other types of bacteria,
particularly bacteria that cause acute
 inflammation such as Staphylococcus and
Streptococcus, and perhaps some fungi.

27. • Antigen-presenting cells
are mostly the dendritic cells which capture the antigens,
transport it to the lymphoid organs and present the
antigens to naïve lymphocytes in order to activate the
immune response.
• Effector cells
of the immune system mainly include activated T
lymphocytes, mononuclear phagocytes, and other
leukocytes. Effector cells are required to complete the
immune cascade, i.e. to eliminate the microbes.

28. Immune Defense against Bacterial Pathogens
Antibody-mediated immunity (AMI)
Cell-mediated immunity (CMI)