Download Adaptive or acquired immune system

Lec-4
Immunology
Dr. Twana A. Mustafa
The immune system consists of two overlapping compartments:
1. Innate immune system
 Most primitive type of immune system; found in virtually all multicellular
animals. Always present and active.
 Nonspecific; not specifically directed against any particular infectious agent or
tumor
 Same every time; no ‘memory’ as found in the adaptive immune system
 First line of defense against infection
 Includes:
1. Physical barriers (skin, mucus lining of gastrointestinal, respiratory and genitourinary
tracts)
2. Phagocytic cells – neutrophils, macrophages
3. Protective chemicals – acid pH of stomach, lipids on skin surface
4. Enzymes – lysozyme in saliva, intestinal secretions; digests cell walls of bacteria
5. Alternate complement pathway – cascade of serum proteins that are activated by
bacterial cell wall components
2. Adaptive or acquired immune system:
 Found only in vertebrates (fish, amphibians, birds and mammals), Must be
induced to be active against infections or tumors.
 Antigen-specific – adaptive immune responses recognize antigens, which can be
proteins, carbohydrates, lipids and nucleic acids.
 Memory – response against a given antigen is much stronger after the first
(primary) response. This heightened reactivity is called secondary responses, and
is due to increased numbers of memory B and T cells to that antigen
 Regulation – discriminates between self and non-self, prevents autoimmune
reactions in most individuals:
- B lymphocytes – differentiate into plasma cells that produce antibodies
- T lymphocytes – subdivided into CD4+ and CD8+ populations
 Helper activity – help other lymphocytes respond to antigen
 Delayed type hypersensitivity – activate macrophages to phagocytose, kill
pathogens
 T cell-mediated cytotoxicity – cytotoxic T cells bind to and kill target cells
(e.g. virus-infected cells and tumor cells)
 Suppressor T cells/Treg cells – down-regulate the responses of other
lymphocytes
The antigenic specificity of the adaptive immune system is due to antigen specific receptors.
1. Immunoglobulins (also called antibodies) – produced by B cell lineage
 IgM, IgD, IgG, IgA, and IgE subtypes
 Surface immunoglobulin (Ig) – antigen-specific receptor of B lymphocytes
 Secreted immunoglobulin (Ig) – Ig molecules secreted by plasma cells
2. T cell receptor (TCR) – antigen-specific receptor of T lymphocytes
- The basic reaction in immunology is the binding of antigen to an antigen-specific
receptor. The affinity of this interaction is similar to that of an enzyme binding to its
substrate.
Ag + Ab =====AgAb
- Typically, each antibody or T cell receptor molecule recognizes a single epitope, a small
region (e.g. 6-10 amino acids) of an antigen.
- In a given B- or T-cell, the antigen-specific receptors of all are identical.
- Each B cell and T cell has its own antigenic specificity, determined by the amino acid
sequence of its surface Ig or TCR. The region of the Ig or TCR that binds to the antigen
is called the paratope.
- In each person, there are ~106 to 108 different Igs and TCRs, giving rise to an almost
endless supply of antigenic specificities. This is called diversity.
The adaptive immune system has memory, meaning that the response against an antigen
is much greater after the first exposure.
 The first response to an antigen is called the primary response, and responses thereafter
are called secondary responses.
 The different properties of secondary responses are due to memory cells generated
during the primary responses. Secondary responses have:
- Higher antibody levels
- Increased proportion of IgG and other immunoglobulin isotypes
- Shorter lag period
- Higher affinity for antigen
Vaccination is effective because it primes the immune system to provide secondary responses
when the individual is exposed to an infectious agent.
 Each exposure to an antigen tends to increase the secondary response. This is why
booster immunizations are often used in vaccinations.