HYPERSENSITIVE REACTION (HSR) 

Immune response that have deleterious effect resulting in tissue damage are called as hyper sensitivity. The reaction  within humoral branch are called as immediate hypersensitivity which manifest its symptoms within minutes.
These are of 3 types-
1. Type I          2. Type II                 3. Type III

Hypersensitivity reaction with in cell mediated branch show its symptoms to a delay of two days following Ag exposure. *It is delayed type hypersensitivity.


Immediate Hypersensitivity-
In immediate hypersensitivity reaction different isotypes induce different immune effector molecules.
 IgE induces mast cell degranulation
 IgM & IgG induces hypersensitivity reaction by activating complement system.

Type I : initiation time 2 to 30 min
In this type hypersensitivity IgE molecules bound to mast cells & basophils get sensitized by cross linking through allergen molecules which induces degranulation in respective cell. Upon degranulation of both Mast cell and basophils, they secrete various allergic mediators.
Receptors for IgE binding on mast cells and basophils are of two types:-
1. Fc εR  I --- high affinity     2. Fc εR II---- low affinity

Type II HSR
This is also called as antibodies mediated cytotoxic HSR.
In this reaction cytotoxic cells with Fc receptors bind to Fc portion of antibodies on target cell and promote the killing. Binding of antibodies to the surface of the cell can result in:
o Phagocytosis of the cell
o Lysis of the cell
o Damage to molecules on the cell surface (e.g., myasthenia gravis)
o Activation of cell-surface receptors (e.g., thyrotoxicosis)
Initiation time for such reaction is 5 to 8 hrs.

Type III HSR
Whenever the antigen- antibody complexes get deposited very near to site of antigen entry or these complexes flow in blood, they will develop localized and systemic reaction respectively. In such reaction complement split products will causes Mast cell degranulation and consequent increase in local vascular permeability.
Such type of reaction will take 2-8 hrs to get initiated.


Type IV Delayed type of reaction:---  such type of HSR has initiation time – 24 to 72 hrs. These are the important reaction against bacteria and parasites that can live intra cellular. In this HSR cell mediated branch of immune system will sensitized Tdth  which will induce release of various cytokines. The net effect of these cytokines is to cause an accumulation and activation of macrophages which will leads to localized non specific destruction of cell and so intracellular pathogens are also get cleared.

THE COMPLEMENT SYSTEM

The complement system plays a major role in host defence and the inflammatory process.
Complement consists of a complex series of at least 15 proteins that normally are functionally inactive in plasma. They are present in normal serum and are thus a part of the innate immunity defences of the body.
1. Complement cascade:  Complement  is activated sequentially in a cascading manner. Each protein activating the protein that directly follows it in the sequence. Complement proteins are not antibody, but the cascade is activated by antibody.
2. Activation of the complement cascade: Have widespread physiologic and pathophysiologic effects. It causes lysis of erythrocytes in haemolytic anemia, sensitizes foreign particles to phagocytosis and causes release of histamine from mast cells.

Features of the system

 The complement system consists of some 30 proteins circulating in blood plasma.
 Most of these are inactive until
 They are cleaved by a protease which, in trun.
 Converts them into a protease
 Thus many components of the system serve as the substrate of a prior component and then as an enzyme to activate a subsequent component.
Complement proteins begin to appear in fetal circulation during the first 13 weeks gestation and are present in neonate at 50% to 60% of adult levels.
The presence of complement proteins before antibody in fetal development suggests that these proteins were the main protection against microbes before the evolutionary development of antibodies.

Three pathway of complement activation:

1) Classical pathway
2) The alternative pathway
3) Lectin pathway

CLASSICAL PATHWAY

The classical pathway triggered by activation of the C1 complex.
1). The C1 complex  is trimolecular  complex. it contains three polypeptides- C1q, C1r,and C1s held together by calcium ions. With the removal of the calcium. C1 breaks down into its three subunits and lose activity.
A. C1q is the portion of the C1 molecule that attaches first to immunoglobulin and initiates complement activation.
B. C1r, when activated , cleaves the proenzyme C1s
C. C1s, when , activated, becomes another serine protease, referred to as C1 esterase because of its esterase activity.
2). C1s cleaves C4 into C4a and C4b.
3).Once bound to the C4b molecule. The next component becomes susceptible to enzymatic attack by the C1s serine protease.
 C2b is activated to become the third serine protease in cascade. It substrates are C3 and Cs.
 Formation of the C4b2a complex, (C3 convertase) requires magnesium ions.
Circulating C3 binds to C4b portion of C4b2a complex to make C5 convertase (C4b2a3b complex). Which is the first component of membrane attack pathway.

complement pathway

MONOCLONAL ANTIBODIES

An antibody produced by a single clone of cells (specifically, a single clone of hybridoma cells ) and therefore a single pure homogeneous type of antibody. The term “monoclonal” pertains to a single clone of cells, a single cell and the progeny of that cell.
Köhler and Milstein found a way to combine
• The unlimited growth potential of myeloma cells with
• The predetermined antibody specificity of normal immune spleen cells.
They did this by literally fusing myeloma cells with antibody-secreting cells from an immunized mouse. The technique is called somatic cell hybridization. The result is a hybridoma.

PROCEDURE

Mix the following spleen cell from a mouse that has been immunized with the desired antigen with myeloma cells.
Use an agent to facilitate fusion of adjacent plasma membrane like PEG. The success rate is so low that there must be a way to select for the rare successful fusions.
• Lost the ability to synthesize hypoxanthine-guanine-phosphoribosyltransferase(HGPRT).
This enzyme enables cells to synthesized purine using an extracellular source of hypoxanthine as a precursor. The absence of HGPRT is not a problem for the cell because cells have an altenate pathway that they can use to synthesize purines.

However, when cells are exposed to aminopterin (a folic acid analog), they are unable to use this other pathway and are now fully dependent on HGPRT for survival.
• Lost the ability to synthesized any antibody molecules of their own (so as not to produce a hybridoma producing two kinds of antibody molecules).

The first property is exploited by transferring the cell fusion mixture ta a culture medium called HAT medium because it contains:
• Hypoxanthine
• Aminopterine
• Pyrimidine thymidine
The logic:
 Unfused myeloma cells cannot grow because they lack HGPRT.
 Unfused normal spleen cells cannot grow indefinitely because of their limited life span.
 Hybridoma cells (produced by successful fusion) are able to grow indefinitely because the spleen cell partner supplies HGPRT and the myeloma partner is immortal.

MHC (Major histocompatibility complex)

These are protein cluster encoded by genes families which have allelic specificity in every person and responsible for presenting proteins at the surface of cells.
These molecules play a major role in determining whether transplanted tissue will be accepted as self or rejected as foreign.

Class 1 MHC :- 

They contain a large α-chain associated noncovalently with the much smaller β-microglobulin molecules. α-chain is organized into three domain (α₁ α₂ and α₃) each containing 90 Aa, a transmembrane domain of  about 40 Aa and a cytoplasmic anchor segment of 30 Aa.

α₃ domain appears to be highly conserved among class 1 MHC molecules and contain a sequence recognized by CD8 molecule present on T- cells.

Class 1 MHC molecules

Class 2 MHC :- 

They contain different polypeptide chain 33 KDa α and 28 KDa β chain which associate by non-covalent interaction. Each chain contains two external domains α₁ and α₂ and β₁, β₂ domains.
α₂ and β₂ contain a sequence recognized by CD4 molecules.

Class 2 MHC molecules

ANTIBODIES (IMMUNOGLOBINS)

ANTIBODIES (IMMUNOGLOBINS)

 are a heterogeneous group of proteins that contain carbohydrates.
 Have sedimentation coefficients ranging from 75-195.
 Are found predomination in the gamma globulin fraction of serum;  some antibodies are also found in α and β globulin fractions.
 Consist of polypeptides chains linked by disulfide bonds, such that each antibody contains a minimum of two heavy (H) chains and two identical light (L) chains.
 Have interchain disulfide bonds holding the chain together (i.e., L to H and H to H).
 Have antigen-binding capacity defined by their specific H and L chains.

ANTIGENS (Immunogens)

ANTIGENS (Immunogens)

• Immunogenicity the capacity production of specific protective humoral or cellular immunity.
• Specific reactivity the capacity to be recognized by the antibodies and T cell producd.
• Foreignness the recognition of a body as nonself (foreign proteins are excellent antigen).
• Size must be at least approximately 10 kilo Daltons to be recognized.
• Shape tertiary and quaternary structure determines the extent of antigenicity.
• Epitope part of Ag that react with BCR and TCR.
-is the restriction portion of antigen molecule that determines the specificity of the reaction with an antibody.
-is the antibody-binding site on an antigen for a specific antibody.
-generally contains four to six amino acid or sugar residues.
• Hapten are Ag but not immunogens
-is a small foreign  molecules that is not immunogenic by itself but can bind to an antibody molecule already formed to it.
-can be immunogenic if coupled to a sufficiently large carrier molecule.
Example- DNP, PAM, IAA.

IMMUNOLOGY

IMMUNOLOGY

Deals with fighting with odds
-Stress, Infection, Disturbance in homeostasis, Graft of foreign or non self-tissue.

IMMUNE SYSTEM

Innate immunity

- is present from birth and non-specific.

- consists of various barriers to external insults; for example, skin, mucous membranes, macrophages,    monocytes, neutrophils, eosinophil, and the contents of these cells. 

Acquired immunity

- is expressed after exposure to given substances and specific.

- involve specific receptors on lymphocytes and the participation of macrophages for its expression.

- consists of:

      * Humoral immunity, mediated by antibodies.

      * Cell mediated immunity, mediated by lymphocytes.

Immunity flow chart

What is the Innate Immune Response?

A universal and evolutionarily mechanism host defence against.

• First line of defence

• Predates the adaptive immune response

• Adaptive only in very rare

• Uses receptors and effectors that are ancient in their lineage

• Must provide protection against a wide variety of pathogens

• Distinguishes self from non self perfectly

• Defects in innate immunity are very rare and almost none.

                                                                    

                                 

 ACQUIRED IMMUNITY

Humoral Immunity                                                                cell mediated Immunity

# Soluble Ag react with Immune                                                               # Insoluble Ag/ part of pathogen take up 

system in solution (blood)                                                                           by Immune cell processed and present on

                                                                                                                          their surface to react with effector cells.

B- cells                                                                                             Process Ag

Complement system                                                                       

Abs                                                                    1.  Endogenous                            2.Exogenous

Phagocytic cells                                     Process by cytoplasmic pathway        endocytic pathway