The document discusses the acquired immune response, focusing on humoral and cell-mediated immunity. Humoral immunity involves the production of antibodies by B lymphocytes, which can be activated by T cells in a T-dependent response or directly without T cells in a T-independent response. Key characteristics of the acquired immune responses include diversity, specificity, long-term memory, and the differentiation between self and non-self antigens.

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.