Lecture outline
• The nomenclature of Immunology
• Types of immunity (innate and adaptive;
active and passive; humoral and cellmediated)
• Features of immune responses
• The major cells of the immune system
Why the great interest in Immunology?
• Basic science: understanding a complex
biological system
• Impact on many aspects of human disease
• Immunization is the ONLY approach for
eradicating a disease
• New therapies based on biology
• Potential for major role in emerging
therapies (gene therapy, stem cell therapy)
Definitions
• Immunity: protection against infections
• Immune system: molecules, cells and tissues that
mediate responses to foreign substances
• Antigens: substances recognized by the cells and
molecules of the immune system and to which the
system responds
The functional importance of the immune system
The central questions
• How does the immune system respond to different
infections?
– Different types of microbes are eliminated by
different effector mechanisms, which are
designed to best combat each type of microbe
– Microbes and hosts are engaged in an
evolutionary struggle, each trying to overcome
the other; many of the features of the immune
system reflect the consequences of such
evolution
The central questions
•
How does the immune system respond to different infections?
– Different types of microbes are eliminated by different
effector mechanisms, which are designed to best combat each
type of microbe
– The evolutionary struggle between microbes and hosts
• Why does the immune system not respond to self
antigens?
• What are the pathogenic mechanisms and clinicopathologic consequences of abnormalities in the
immune system?
Innate and adaptive immunity
Innate immunity: always present (ready to attack); many pathogenic
microbes have evolved to resist innate immunity
Adaptive immunity: stimulated by exposure to microbe; more potent
Properties of adaptive immune responses
The two features that best distinguish adaptive and
innate immunity are specificity and memory
Primary and secondary immune responses illustrate
specificity and memory in adaptive immunity
The concept of clonal selection
Lymphocytes with highly specific and diverse antigen receptors
develop prior to exposure to antigens
Active and passive immunity
Active immunity: long-lasting protection (memory),
multiple effector mechanisms activated, lag time
Passive immunity: rapid protection, short duration
Cells of the immune system
• Lymphocytes
– Mediators of adaptive immune responses; only
cells with specific receptors for antigens
• Antigen-presenting cells (APCs)
– Specialized to capture, concentrate, and display
antigens for recognition by lymphocytes
– Dendritic cells; macrophages, B cells; follicular
dendritic cells
• Effector cells
– Function to eliminate microbes; include
lymphocytes, granulocytes (neutrophils,
eosinophils), macrophages
Development of B and T lymphocytes
Congenital immunodeficiency diseases are often caused by blocks
at different stages of lymphocyte maturation
Classes of lymphocytes
The CD Nomenclature
•
Structurally defined leukocyte surface molecule that
is expressed on cells of a particular lineage
(“differentiation”) and recognized by a group
(“cluster”) of cell-specific antibodies is called a
member of a cluster of differentiation (CD)
•
CD molecules (CD antigens, CD markers) are:
•
•
•
Used to classify leukocytes into functionally distinct
subpopulations, e.g. helper T cells are CD4+CD8-, CTLs
are CD8+CD4Often involved in leukocyte functions
Antibodies against various CD molecules are used to:
•
•
•
Identify and isolate leukocyte subpopulations
Study functions of leukocytes
Eliminate particular cell populations
Types of adaptive immunity
Different types
of immune
responses are
mediated by
different classes
of lymphocytes
and defend
against different
types of microbes
Phases of adaptive immune responses
Need for proliferation and differentiation results in delay
(typically 4-7 days) in the adaptive immune response
Stages in the life history of lymphocytes
Proliferation: expands number of antigen-specific cells
Differentiation: converts lymphocytes into effective defenders
Naïve, effector and memory lymphocytes
• Naïve lymphocytes
– Mature lymphocytes that have not previously
encountered antigen; function -- antigen
recognition
– Preferential migration to peripheral lymphoid
organs (lymph nodes), the sites where antigens
are concentrated and immune responses start
Naïve, effector and memory lymphocytes
•
Naïve lymphocytes
– Mature lymphocytes that have not previously encountered antigen;
function -- antigen recognition
– Preferential migration to peripheral lymphoid organs (lymph
nodes), the sites where antigens are concentrated and immune
responses start
• Effector lymphocytes
– Activated lymphocytes capable of performing the
functions required to eliminate microbes
(‘effector functions”)
– Effector T lymphocytes: cytokine secretion
(helper cells), killing of infected cells (CTLs)
– B lymphocytes: antibody-secreting plasma cells
Naïve, effector and memory lymphocytes
•
•
Naïve lymphocytes
– Mature lymphocytes that have not previously encountered antigen;
function -- antigen recognition
– Preferential migration to peripheral lymphoid organs (lymph
nodes), the sites where antigens are concentrated and immune
responses start
Effector lymphocytes
– Activated lymphocytes capable of performing the functions
required to eliminate microbes (‘effector functions”)
– Effector T lymphocytes: cytokine secretion (helper cells), killing
of infected cells (CTLs)
– B lymphocytes: antibody-secreting cells (e.g. plasma cells)
• Memory lymphocytes
– Long-lived, functionally silent cells; mount rapid
responses to antigen challenge (secondary
responses)