Overview of the Immune System Innate (Nonspecific)

Acquired immunity
The immunity acquired during the lifetime of an individual is known as acquired immunity. Acquired immunity differs from innate immunity in the following respects:
It is not inherent in the body but is acquired during life.
It is specific for a single type of microorganism.

ANTIGEN is a substance which, when introduced parenterally into the body, stimulates the production of an antibody with which it reacts specifically in an observable manner.
The main property of the antigen is immunogenicity.

Factors Influencing Immunogenicity
Foreignness - only antigens which are foreign to the individual induce an immune response
Size - usually antigens have a molecular weight of 10,000 or more (20-30 kD).
Susceptibility to tissue enzymes: Only substances which are metabolized and are susceptible to the action of tissue enzymes behave like antigen.
Chemical Composition
Primary Structure
Secondary Structure
Tertiary Structure
Quarternary Structure

Chemical Nature of Immunogens
Proteins
Polysaccharides
Nucleic Acids
Lipids
Some glycolipids and phosopholipids can be immunogenic for T cells

Types of Antigens
On the basis of capability of antibody formation antigens are classified as under:
T-cell independent antigens
T-cell dependent antigens

Types of Antigens
T-independent
Polysaccharides
Properties
Polymeric structure
Polyclonal B cell activation
Resistance to degradation
Examples
Pneumococcal polysaccharide, Lipopolysaccharide
Flagella

Types of Antigens
T-dependent
Proteins
Structure
Examples
Microbial proteins
Non-self or Altered-self proteins

Types of Antigens
Depending on the size and capacity to induce antibody production antigens can be divided into:
Complete antigen. It is able to induce antibody formation and produce a specific and observable reaction with the antibody so produced, e.g. proteins.
Partial antigen (also called hapten): Haptens are substances which are unable to induce antibody production by themselves, but are able to react specifically with antibodies, e.g. lipids, nucleic acid, sulfonamide, penicillin, etc.

Superantigens
Monoclonal/Oligoclonal T cell response
Definition – they can induce very powerful immune response (examples - Staphylococcal enterotoxins, Streptococcal pyrogenic exotoxins)

Antibody is humoral substance (γ-globulin) produced in response to an antigenic stimulus.
Antibodies are:
Protein in nature.
Formed in response to antigenic stimulation.
React with corresponding antigen in a specific and observable manner.

Antibodies
Immunoglobulins are synthesized by plasma cells.
Immunoglobulins make 20-25% of the total serum proteins.
Five groups of immunoglobulins have been distinguished: IgG, IgA, IgM, IgD and IgE.

Affinity =  attractive and repulsive forces
Affinity
Strength of the reaction between a single antigenic determinant and a single Ab combining site

Avidity
The overall strength of binding between an Ag with many determinants and multivalent Abs

Specificity
The ability of an individual antibody combining site to react with only one antigenic determinant.
The ability of a population of antibody molecules to react with only one antigen.

Cross Reactivity
The ability of an individual Ab combining site to react with more than one antigenic determinant.
The ability of a population of Ab molecules to react with more than one Ag

Factors Affecting Measurement of Ag/Ab Reactions
Affinity
Avidity
Ag:Ab ratio
Physical form of Ag

Tests Based on Ag/Ab Reactions
All tests based on Ag/Ab reactions will have to depend on lattice formation or they will have to utilize ways to detect small immune complexes
All tests based on Ag/Ab reactions can be used to detect either Ag or Ab

Agglutination/Hemagglutination
Definition - tests that have as their endpoint the agglutination of a particulate antigen
Agglutinin/hemagglutinin

Agglutination/Hemagglutination
Definition
Qualitative test
Quantitative test
Applications
Blood typing
Bacterial infections
Fourfold rise in titer
Practical considerations
Easy
Semi-quantitative

Passive Agglutination/Hemagglutination
Definition - agglutination test done with a soluble antigen coated onto a particle
Applications
Measurement of antibodies to soluble antigens

Coombs (Antiglobulin)Tests
Incomplete Ab
Direct Coombs Test
Detects antibodies on erythrocytes
Applications: 1. Detection of anti-Rh Ab
2. Autoimmune hemolytic anemia

Coombs (Antiglobulin)Tests
Indirect Coombs Test
Detects anti-erythrocyte antibodies in serum

Agglutination/Hemagglutination Inhibition
Definition - test based on the inhibition of agglutination due to competition with a soluble Ag
Applications: Measurement of soluble Ag

Precipitation reactions
When a soluble antigen combines with its antibody in presence of electrolytes (NaCl) at a suitable temperature and pH the antigen antibody complex forms insoluble precipitate.
Uses of precipitation reaction
Identification of bacteria
Identification of antigenic component of bacteria in infected animal tissue (Ascoli test)
Standardization of toxin and antitoxins
Demonstration of antibody in serum
Medicolegal serology for detection of blood, serum, etc.

TECHNIQUES OF PRECIPITATION REACTION
Slide test. When a drop of antigen and antiserum is placed on a slide and mixed by shaking, floccules appear.

Ring test. The antigen is layered over serum in a narrow tube. The reaction is visible as a white zone at the junction of two clear fluids.

TECHNIQUES OF PRECIPITATION REACTION
Tube test. The Kahn test for syphilis is an example of tube flocculation test.
Gel diffusion. The precipitate is relatively fixed by agar medium and is easily visible.

Competitive RIA/ELISA for Ag
Method
Determine amount of Ab needed to bind to a known amount of labeled Ag
Use predetermined amounts of labeled Ag and Ab and add a sample containing unlabeled Ag as a competitor

Immunofluorescence
Direct
Ab to tissue Ag is labeled with fluorochrome

Immunofluorescence
Indirect
Ab to tissue Ag is unlabeled
Fluorochrome-labeled anti-Ig is used to detect binding of the first Ab.
Qualitative to Semi-Quantitative