1. The Immune System Part D
Chapter 22
The Immune System Part D

2. Adaptive (Specific) Defenses
Is antigen-specific, systemic, and has memory
Has two separate but overlapping components:
Humoral, or antibody-mediated immunity
Cellular, or cell-mediated immunity

3. Are substances that can:
Antigens
Are substances that can:
Mobilize the immune system
Provoke an immune response
Are the ultimate targets of all immune responses
Are mostly large, complex molecules
Not normally found in the body (nonself)

4. Adaptive (Specific) Defenses
The adaptive (specific) defense system represents:
The 3rd line of defense
Works in conjunction with the innate system
Recognizes & attack particular foreign substances
Acts to:
Immobilize
Neutralize
Destroy foreign substances

5. Complete Antigens
Complete antigens have two important functional properties:
Immunogenicity:
Ability to stimulate:
Proliferation of specific lymphocytes
Antibody production
Reactivity:
Ability to react with:
Activated lymphocytes
Antibodies released in response to them

6. Complete Antigens, cont’d.
Complete antigens include:
Foreign proteins (strongest)
Nucleic acids
Some lipids
Large polysaccharides

7. Incomplete Antigens (Haptens)
Small molecules, such as:
Peptides
Nucleotides
Many hormones
Not immunogenic
When attached to body proteins:
Become reactive
Recognized as foreign by the adaptive immune system
Attacked by the system (resulting in allergy or hypersensitivity reactions)
Found in:
Poison ivy, animal dander, some detergents, cosmetics
Some household & industrial products

8. Antigenic Determinants
Only certain parts of an antigen are immunogenic
These parts are called antigenic determinants
Antibodies & activated lymphocytes bind to these antigenic determinants
Most naturally occurring antigens have numerous antigenic determinants that a single antigen may:
Mobilizes several different lymphocyte populations
Forms different kinds of antibodies
Large, chemically simple molecules (e.g., plastics) have little or no immunogenicity

9. Antigenic Determinants
Figure 21.7

10. Self-Antigens: Major Histocompatibility Complex (MHC) Proteins
Our cells are dotted with protein molecules
These molecules are:
Defined as “self antigens”
Not antigenic to us, but
Strongly antigenic to others
One type (glycoproteins) is called:
Major histocompatibility complex (MHC) proteins

11. Self-Antigens: MHC Proteins
This group of glycoproteins marks a cell as self
It is coded for by millions of gene combinations
Unlikely two people (except identical twins) have the same MHC proteins
There are two classes of MHC proteins:
Class I MHC proteins:
Found on virtually all body cells
Class II MHC proteins:
Found only on certain cells
Such cells act in the immune response

12. Cells of the Adaptive Immune System
There are three crucial cell types of this system:
Two types of lymphocytes
A 3rd type of antigen-presenting cells
Lymphocyte types:
B lymphocytes
Oversee humoral immunity
T lymphocytes:
Non-antibody-producing cells
Constitute the cell-mediated arm of immunity
Antigen-presenting cells (APCs):
Do not respond to specific antigens
Play essential auxiliary roles in immunity

13. Lymphocyte “Education”
Lymphocytes originate in red bone marrow
During development lymphocytes are “educated”
The aim of this education is twofold:
Immunocompetence:
Each lymphocyte must become able to recognize its one specific Ag by binding to it
Self-tolerance:
Each lymphocyte must be relatively unresponsive to self-antigens and doesn’t attack the body’s own cells

14. Lymphocyte Maturation
Immature lymphocytes released from bone marrow are essentially identical
Lymphocyte maturation “education” into B or T cells:
Depends on location of competence development
B cells become immunocompetent in bone marrow
T cells migrate (in blood) to the thymus & mature there
The thymus & bone marrow (competence locations) are called:
The primary lymphoid organs
All other lymphoid organs are called
The secondary lymphoid organs

15. T cells mature in the thymus under two types of selection pressures:
Negative selection:
Eliminates T cells that are strongly anti-self
Positive selection:
Selects T cells with a weak response to self-antigens
Thus positively selected T cells become both:
Immunocompetent, and
Self-tolerant

16. In bone marrow, B cells become:
Immunocompetent, and
Self-tolerant in bone marrow
Self-reactive B cells are either:
Eliminated by apoptosis
Let undergo receptor editing (receptor rearrangement)
Some do leave bone marrow & get inactivated peripherally

17. Antigen-Presenting Cells (APCs)
Major roles of APCs in immunity:
Engulf foreign particles (antigens)
Present antigen fragments on their own surfaces
Flag T cells to recognize antigens
Major APCs are:
Dendritic cells (DCs)
Macrophages
Activated B cells

18. Antigen-Presenting Cells (APCs)
Dendritic cells (DCs):
The major initiators of adaptive immunity
Migrate to lymph nodes & other 2ndary lymphoid organs
Present antigens to T and B cells

19. Macrophages and Dendritic Cells
Secrete soluble proteins that activate T cells
Activated T cells in turn release chemicals that:
Speed up the maturation & mobilization of dendritic cells
Prod macrophages to become activated macrophages
Activated macrophages:
Are insatiable phagocytes
Secrete bactericidal chemicals

20. Humoral Immunity Response
Antigen challenge:
Refers to the 1st encounter between an antigen and a naive immunocompetent cell
Takes place usually in the spleen or a lymph node
May happen in any other 2ndry lymphoid organ
If the challenged lymphocyte is a B cell:
The antigen provokes a humoral immune response
Antibodies are produced against the challenger

21. Proliferation to form a clone
Antigen
Primary Response
(initial encounter
with antigen)
Antigen binding
to a receptor on a
specific B lymphocyte
(B lymphocytes with
non-complementary
receptors remain
inactive)
Proliferation to
form a clone
B lymphoblasts
Plasma
cells
Memory
B cell
Secreted
antibody
molecules
Secondary Response
(can be years later)
Clone of cells
identical to
ancestral cells
Subsequent
challenge by
same antigen
Plasma
cells
Secreted
antibody
molecules
Memory
B cells
Figure 21.10

22. Fate of the Clones
Most clone cells become plasma cells
Plasma cells are antibody-secreting cells
They secrete specific antibody
Secretion is at a rate of 2000 molecules per second

23. Immunological Memory: Primary & 2ndary immune response
Primary immune response:
Cellular differentiation & proliferation
Occurs on the first exposure to a specific antigen
A lag period of days after antigen challenge
Antibody plasma levels peak in 10 days
Antibody levels then decline

24. Secondary immune response:
Immunological Memory
Secondary immune response:
Re-exposure to the same antigen
Sensitized memory cells respond within hours
Antibody levels peak in 2 to 3 days
Levels are much higher than in the primary response
Antibodies bind with greater affinity
Blood levels can remain high longer

25. Active Humoral Immunity
Naturally acquired Abs:
Response to a bacterial or viral infection
Artificially acquired:
Response to a vaccine
By using dead or attenuated (weakened) pathogens
Vaccines:
Spare us the symptoms of disease
Their weakened antigens provide:
immunogenic & reactive antigenic determinants

26. Passive Humoral Immunity
Differs from active one in Ab source & degree of protection:
B cells are not challenged by antigens
Immunological memory does not occur
Protection ends when Ab naturally degrade in body
Naturally acquired:
From a mother to her fetus via the placenta
Via milk to infants
Artificially acquired:
From serum injection
Example: Gamma globulin

27. Types of Acquired Immunity
Figure 21.12

28. Also called immunoglobulins
Antibodies
Also called immunoglobulins
Constitute the gamma globulin portion of blood proteins
Are soluble proteins secreted by plasma cells in response to an antigen
Are capable of binding specifically with that antigen
There are five classes of antibodies:
IgG, IgM, IgA, IgD, IgE

29. Antibodies themselves:
Antibody Targets
Antibodies themselves:
Do not destroy antigen
They inactivate & tag it for destruction
All form antigen-antibody (immune) complex
Defensive mechanisms used by antibodies include:
Neutralization,
Agglutination
Precipitation
Complement fixation