This document provides an overview of adaptive immunity, including: - The cells that participate including T cells, B cells, and antigen presenting cells - The clonal selection theory which explains how lymphocytes proliferate in response to antigens - The mechanisms of the immune response when exposed to antigens, including activation of B and T cells and development of memory cells - Methods of acquiring immunity, including actively through natural infection/vaccination or passively through breast milk/placenta

1. Adaptive Immunity
Prepared by: Ahmad Hasan, Rasul Akram, Nahro Rostam, Muhammed
Erfan, Sham Muhammed, Anas Adnan.
Supervised by: Phd. Trefa Muhammed Abdullah
College of Pharmacy : stage 2 5/9/23 Immunology

2. Contents
Introduction ..............................................1
The cells participating in the immune
response....................................................1
T cells and APCs ....................................1
B cells.....................................................1
Clonal Selection Theory ............................2
Immune Response Mechanism.................2
Methods of Acquiring Immunity...............3
Active.....................................................3
Passive ...................................................3
References ................................................4
Introduction
The primary functions of the adaptive immune response are: the recognition of specific “nonself”
antigens, distinguishing them from “self” antigens; the generation of pathogen-specific immunologic
effector pathways that eliminate specific pathogens or pathogen-infected cells; and the development of an
immunologic memory that can quickly eliminate a specific pathogen should subsequent infections occur.
Adaptive immune responses are the basis for effective immunization against infectious diseases.
The cells participating in the immune response
The cells of the adaptive immune system include: antigen-specific T cells, which are activated to
proliferate through the action of APCs (antigen presenting cells), and B cells which differentiate into
plasma cells to produce antibodies
T cells and APCs
T cells derive from hematopoietic stem cells in bone marrow and,
following migration, mature in the thymus. T cells require the action of
APCs (usually dendritic cells, but also macrophages, B cells, fibroblasts
and epithelial cells) to recognize a specific antigen. The surfaces of APCs
express a group of proteins known as the major histocompatibility
complex (MHC), MHC is either classified as class I or class II. Class I MHC
molecules present endogenous (intracellular) peptides, while class II
molecules on APCs present exogenous (extracellular) peptides to T
cells.
T cells are activated when they encounter an APC that has digested an
antigen and is displaying the correct antigen fragments (peptides)
bound to its MHC molecules.
B cells
B cells arise from hematopoietic stem cells in the bone marrow and, following maturation, leave the
marrow expressing a unique antigen-binding receptor on their membrane. Unlike T cells, B cells can

3. recognize antigens directly, without the need for APCs, through unique antibodies
expressed on their cell surface. The principal function of B cells is the production of
antibodies against foreign antigens, Five major types of antibodies are produced by
B cells: IgA, IgD, IgE, IgG and IgM. IgG. Under certain circumstances, B cells can also
act as APCs
Clonal Selection Theory
The clonal selection theory is a widely accepted model for the immune system’s
response to infection. The theory proposed that:
1. Antibodies and lymphocytes of myriad specificities exist before there is any
contact with the foreign antigen.
2. The lymphocytes participating in the immune response have antigen-specific receptors on their surface
membranes. In the case of B lymphocytes, the receptors are molecules bearing the same specificity as the
antibody which the cell will subsequently produce and secrete.
3. Each lymphocyte carries on its surface receptor molecules of only a single specificity.
4. Immunocompetent lymphocytes,
combining with the foreign antigen by virtue
of their surface receptors, are stimulated
under appropriate conditions to proliferate
and differentiate into clones of cells making
antibody (immunoglobulins or Ig) of that
particular specificity. It should be noted that
if several distinct regions of an antigen can
be recognized, several different clones of
cells will be stimulated to produce antibody,
the sum total of which would represent an
antiserum specific for that antigen but made
up of antibodies of differing specificity.
5. Circulating "self" antigens that reach the
developing lymphoid system prior to some
undesignated maturational step will serve to
shut off those cells that recognize it
specifically and no subsequent immune
response will be induced
Immune Response Mechanism
When an individual is exposed to non-self substance either by injection or infection, a complex series of
events are created: An antigen-presenting cell (usually a macrophage) processes the antigen and presents
it to the lymphoid cells of the immune system
• For a successful immune response to occur, the processed antigen (specifically, its epitope) must be
presented to lymphocytes in association with a glycoprotein encoded by genes of the major
histocompatibility complex (MHC).

4. • This requirement for effective cell interaction is called MHC restriction.
• The lymphoid cells recognize that particular epitope and acquire the ability to react with it.
The result of these consequences of events is the activation of antigen-specific B and T cells, causing them
to proliferate and mature
The consequences of the initial interaction between lymphocytes and their homologous epitopes are far-
reaching.
⁃ A subsequent exposure to antigen will induce some B lymphocytes (memory B cells) to proliferate and
differentiate into antibody-secreting plasma cells.
• These active plasma cells secrete their specific antibody in large amounts when they contact antigen a
second time, a phenomenon known as anamnesis.
• The secreted antibodies react specifically with the antigen that originally induced the B cell to
proliferate. The potential exists to produce an extremely large (> 100,000) variety of different, specifically
reactive, antibodies.
• Some T lymphocytes (memory T cells) are induced to differentiate and proliferate to form mature
progeny that will be triggered to release biologically active metabolites when they contact antigen a
second time.
Methods of Acquiring Immunity
Active
1. Naturally: immunity gained from illness and recovery.
2. Artificially: immunity gained from vaccine
Passive
1. Naturally : Immunity acquired from antibodies passing
through breast milk or through placenta.
2. Artificially: Immunity gained from antibodies harvested
from another person or animal

5. References
• LibreTexts. (n.d.). Clonal Selection of Antibody-Producing Cells. Retrieved 6th, May 2023 from:
https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_(Boundless)/
11:_Immunology/11.07:_Antibodies/11.7C:_Clonal_Selection_of_Antibody-Producing_Cells
• Marshall, J. (2018). An introduction to immunology and immunopathology. Retrieved 6th, May 2023
from: https://aacijournal.biomedcentral.com/articles/10.1186/ s13223-018-0278-1#:~:text=Numerous
cells are involved in,cell types: neutrophils and macrophages.