Adaptive immunity involves specialized immune cells and antibodies that attack and destroy foreign invaders and are able to prevent disease in the future by remembering what those substances look like and mounting a new immune response.

2. Immunity and immune system
The acquired immune response
I- Humoral Immune response
by: Dr
Mohamed. M. Amin
Associate professor of microbiology and immunology
Faculty of medicine - Aswan University
Egypt

3. • It is an inducible, specific immunologic response to exposure to a particular infectious
agent.
• It may be:
1. Humoral immunity: It is an immune state resulting from the production of antibodies by
bone marrow derived B Lymphocytes (plasma cells)
2. Cell-mediated immunity (cellular):It is an immune response primarily of thymus
derived T lymphocytes which inhibits organisms such as fungi, intracellular bacteria,
virus-infected cells and tumor cells
Characters of the acquired immune responses:
• Diversity (i.e. they can respond to millions of different antigens)
• Specificity (i.e. their actions are specifically directed against the antigens that initiated the
response)
• Long memory (T & B memory cells) can respond many years after initial exposure to
certain Ag
• Differentiation between self and non-self antigens
The acquired immune response

5. Humoral Immune response
• Definition: it is the production of antibodies
by plasma cells (activated B lymphocytes)
with binding of these antibodies to
extracellular pathogen resulting in
elimination of this pathogen with the help of
accessory cells and molecules.

6. HUMORAL IMMUNE RESPONSE
• Activation of B Cells and Production of Antibodies:
– Antibody production to protein antigens introduced
into the body requires the direct contact of B cells
with TH cells and their cytokines. These are called
thymus dependent or T-dependent antigens.
– Antibody responses to non-protein antigens, such as
polysaccharides and lipids, do not require antigen
specific helper T cells for antibody production.
These are called thymus independent or T-
independent antigens.

7. • The process of activation of B cells and the generation of antibody producing
cells in response to protein antigens consists of sequential phases:
1. Mature naive antigen specific B cells, after leaving the bone marrow,
populate the peripheral lymphoid tissues, where they meet their specific
antigens. Antigens are recognized by specific receptors (IgM-IgD) on the
surface of B cells. This provides the first signal of B cell activation. Then the
antigen is internalized by B cells and processed and presented on its
surface in association with MHC II to specific TH cells.
2. B7 molecules are induced on B cells and interact with CD28 on T cells.
This second co-stimulatory interaction is needed for proper antigen
presentation to TH cells and their activation.
3. TH cells activated by antigen and B7 co-stimulation express a surface
molecule called CD40L which interact with CD40 molecule on B cells that
are presenting antigen. This signal is essential for secretion of cytokines
and for immunoglobulin class switching.
4. The direct contact of B and TH cells and secretion of cytokines by TH cells
(IL-2, 4, 5 and 6) provide the final signal for B cell activation, proliferation
and differentiation to effector plasma cells secreting antibodies.

8. • The cytokines secreted induce immunoglobulin heavy
chain class switching from IgM, which is the first
antibody produced, to IgG, IgA or IgE.
5- Some activated B cells differentiate into long-lived
memory cells that remain quiescent for long periods but
are capable of being activated rapidly upon re-exposure to
antigen. The presence of these memory cells explains the
rapid appearance of antibody in the secondary immune
response. They are also responsible for long-lasting
immunity after some infections e.g. measles and
poliomyelitis.

9. • Non-peptide antigens
• activate B cells directly without the help of T cells and are called
T cell independent (TI) antigens. Such antigens are generally
large polymers with repeating antigenic epitopes, such as
bacterial capsular polysaccharides. The multiple, identical
epitopes can cross-link surface receptors on B cells and stimulate
their proliferation and IgM production.
• In the T cell dependent response all classes of antibodies are
made (IgG, IgM, IgA, IgE) whereas in the TI response only IgM
is made (this indicates that cytokines produced by T cells are
needed for class switching). T cell dependent response generates
memory B cells whereas the TI response does not, so a
secondary antibody response does not occur in the latter.