The acquired immune system consists of cell-mediated and antibody-mediated immunity. Cell-mediated immunity involves T cells like helper T cells and cytotoxic T cells that recognize and destroy infected or abnormal cells. Antibody-mediated immunity involves B cells and antibodies that neutralize toxins, viruses, and opsonize bacteria. Both responses exhibit diversity, memory, and specificity targeting pathogens for destruction through recognition of antigens by immune cells.

1. Acquired Immunity & Antigen
Aman Ullah

2. Acquired Immunity
• The two components of the acquired arm are
1. Cell-mediated immunity
2. Antibody-mediated (humoral) immunity
• The cell-mediated arm consists primarily of T lymphocytes (e.g., helper T
cells and
• cytotoxic T cells)
• The antibody-mediated arm consists of antibodies (immunoglobulins) and
B lymphocytes (and plasma cells)
• Some of the major functions of T cells and B cells:
• The main functions of antibodies are (1) to neutralize toxins and viruses
and (2) to opsonize bacteria making them easier to phagocytize
• Cell-mediated immunity inhibits organisms such as fungi, parasites, and
certain intracellular bacteria such as Mycobacterium tuberculosis
• It also kills virus-infected cells and tumor cells

4. Important features of cell & antibody
mediated immunity
• Cell-mediated and antibody-mediated
responses are characterized by three
important features:
1. They exhibit remarkable diversity
2. They have a long memory
3. They exhibit exquisite specificity

5. Specificity of the immune response
• Cell-mediated immunity and antibody are both
highly specific for the invading organism
• The specificity can be summarized by three
actions:
• Recognition of the foreign organism by specific
immune cells
• Activation of these immune cells to produce a
specific response
• Response that specifically targets the organism
for destruction

6. Example of specific immunity using viral infection as a model

7. Cell-mediated immunity
Helper T lymphocytes :
• M. tuberculosis enters the body and is ingested by a macrophage
• The bacterium is broken down, and fragments of it called antigens
or epitopes appear on the surface of the macrophage in association
with class II major histocompatibility complex (MHC) proteins
• The antigen–class II MHC protein complex interacts with an antigen-
specific receptor on the surface of a helper T lymphocyte
• Activation and clonal proliferation of this antigen-specific helper T
cell occur as a result of the production of interleukins, the most
important of which are interleukin-1 and interleukin-2
• These activated helper T cells, aided by activated macrophages,
mediate one important component of cellular immunity, i.e., a
delayed hypersensitivity reaction specifically against M.
tuberculosis

8. Cell-mediated immunity
Cytotoxic (cytolytic) T lymphocytes:
• Specific effectors of the cellular immune response, particularly
against virus-infected cells
• Virus e.g., influenza virus, is inhaled and infects a cell of the
respiratory tract
• Viral envelope glycoproteins appear on the surface of the infected
cell in association with class I MHC proteins
• A cytotoxic T cell binds via its antigen-specific receptor to the viral
antigen–class I MHC protein complex and is stimulated to grow into
a clone of cells by interleukin-2 produced by helper T cells
• These cytotoxic T cells specifically kill influenza virus–infected cells
(and not cells infected by other viruses) by recognizing viral
antigen–class I MHC protein complexes on the cell surface and
releasing perforins that destroy the membrane of the infected cell

9. Antibody-Mediated Immunity
• Antibody synthesis typically involves the cooperation of three cells:
• macrophages, helper T cells, and B cells.
• After processing by a macrophage, fragments of antigen appear on the
surface of the macrophage in association with class II MHC
• Proteins
• The antigen–class II MHC protein complex binds to specific receptors on
the surface of a helper T cell, which then produces interleukins such as
interleukin-2 (Tcell growth factor), interleukin-4 (B-cell growth factor), and
interleukin-5 (B-cell
• differentiation factor)
• These factors activate the B cell capable of producing antibodies specific
for that antigen
• The activated B cell proliferates and differentiates to form many plasma
cells that secrete large amounts of immunoglobulins (antibodies).
• Certain antigens, e.g., bacterial polysaccharides, can activate B cells
directly, without the help of T cells, and are called T-cell-independent
antigens.

10. Antibody-Mediated Immunity
• In T-cell-independent response, only IgM is
produced by B cells because it requires
interleukins 4 and 5 made by the helper T cell for
the B cell to "class switch" to produce IgG, IgA,
and IgE
• B cells can perform two important functions
during the induction process:
1. They recognize antigens with their surface IgM
that acts as an antigen receptor
2. They present epitopes to helper T cells in
association with class II MHC proteins