IMMUNITY

Antigens:- A molecule that is recognized by cells of the immune system is called an antigen. and many elicit a response from these cells .may it (consist of soluble molecules such as proteins ,polysaccharides ,and nucleoproteins).may antigen belonging to whole cells(bacteria ,protozoa ,tumor cells, or virus –infected cells).the cells of the immune system do not recognize and react to the whole antigen molecule but instead react to small molecular domains of the antigen .known as antigenic determinants or epitopes. Antigen: Substances that can be recognized by the surface antibody (B cells) or by the TCR when associated with MHC molecules



Antibodies
An antibody is a glycoprotein that interacts specifically with an antigenic determinate. Antibody belong to the immunoglobulin(Ig) protein family .it secreted by plasma cells that arise by proliferation and terminal differentiation of clones of B lymphocytes whose receptors recognize and bind specific epitopes. there are a common classes of antibody molecules it consist of two identical light chain and two identical heavy chain ,bound by disulfide bonds .
Classes of Antibodies
1- IgG :is the most abundant class representing 75% of serum immunoglobulin it is produced in large amounts during immune responses. IgG is the only immunoglobulin That crosses the placental barrier and is transported to the circulatory system of the fetus , protecting the new born against infections for a certain period of time.
2-IgA:-is the main immunoglobulin found in secretions , such as nasal bronchial ,intestinal , and prostatic ,as well as in tears ,colostrum ,saliva, and vaginal fluid ,it is present in secretions as admire or timer called secretory IgA. The IgA secretory or transport component because it is resistant to several enzymes.
3-IgM:constitutes about 10% of blood immunoglobulin and usually exists as a pentamer ,together with IgD the major immunoglobulin found on the surface of B lymphocytes secreted IgM ,when bound to antigen , is very effective in activating the ( complement system)
4-IgE :-usually exists as a monomer ,it has a great affinity for receptors present on the surface of mast cells & basophils ,it small amounts are found in the blood stream that lead secretion heparin and histamine this characterizes a low of 0.004% and serum concentration of 0.0003 g / l. It is also renewed every two days and interferes with allergic reactions.
5-IgD:-it properties and activates of IgD are not completely understood, the concentration in blood plasma about 0.2%, Recently, IgD was found to bind to basophils and mast cells and activate these cells to produce antimicrobial factors to participate in respiratory immune defense in humans. It also stimulates basophils to release B cell homeostatic factors. This is consistent with the reduction in the number of peripheral B cells .


Actions of antibodies
Some antibodies are able to agglutinate cells and to precipitate soluble antigens, thus neutralizing their harmful effects on the body .Antigen-antibody complexes and system a-group of around 20 plasma proteins produced mainly in the liver and activated through cascade of reactions.
Cytokines
Cytokines ,which are peptides or glycoproteins with low molecular masses (between 8 and 80 KDa) .cytokines act on many cells that have receptors for them not only cells of the immune system, but also cells of other system such as the nervous system and endocrine system .they are primarily produced by cells of the immune system ,mainly lymphocytes , macrophages & leukocytes , but many also be synthesized by other cell types, such as endothelial cells and fibroblasts. Called chemtaxins or chemokines.
The cytokines produced by macrophages have long-range effects that contribute to host defense, one of these is the elevation of body temperature these cytokines are termed (endogenous pyrogens) because they cause fever and derive from an endogenous source that rather than from bacterial components such as LPS.
Cell of the immune system
The primary cells that participate in the immune response are lymphocytes ,plasma cells ,mast cells ,neutrophils ,eosinophils & cells, of the mononuclear phagocyte system. and antigen presenting cell(APC).
Lymphocytes
Classified as B,T or natural killer cells ,the B and T cells are the only cells that have the ability to selectively recognize a specific epitope among a vast number of different epitopes ,B and T cells differ based on their life history ,surface receptors and behavior during an immune response.
Major histocompatibility complex (MHC)& Antigen presentation.
The major histocompatibility (MHC) is a complex of chromosome loci that encodes several proteins known as class I MHC molecules and class II MHC molecules .Because a great many alleles exist for each of the loci , there is great variation of these molecules among the general population .One individual .however expresses only one set of class I proteins and one are set of class II proteins , these proteins are unique to this individual .All nucleated cells have class I proteins ,class II proteins ,however exist in only a small group of cells operationally denominated (antigen-presenting cells).MHC molecules are integral membrane proteins present on the cell surface .they are synthesized by polyribosomes and are inserted in the membrane of the rough endoplasmic reticulum, as a regular membrane protein however , on their way to the cell surface they couple with small peptides of 10-3- amino acids whose origin differs depending on whether class I or class II molecules are involved.
MHC is the tissue-antigen that allows the immune system (more specifically T cells) to bind to, recognize, and tolerate itself (auto recognition). MHC is also the chaperone for intracellular peptides that are complexes with MHCs and presented to T cell receptors (TCRs) as potential foreign antigens. MHC interacts with TCR and its co-receptors to optimize binding conditions for the TCR-antigen interaction, in terms of antigen binding affinity and specificity, and signal transduction effectiveness.
Essentially, the MHC-peptide complex is a complex of auto antigen / alloantigen. Upon binding, T cells should in principle tolerate the auto-antigen, but activate when exposed to the alloantigen. Disease states occur when this principle is disrupted .
Antigen presentation MHC molecules bind to both T cell receptor and CD4/CD8 co-receptors on T lymphocytes, and the antigen epitope held in the peptide-binding groove of the MHC molecule interacts with the variable Ig-Like domain of the TCR to trigger T-cell activation.
Antigen-presenting cells (APCs)
APCs are found in many tissues and constitute a heterogeneous cell population that includes B lymphocytes ,macrophages , and dendritic cells .the dendritic cells(not to be confused with cells of the nerves tissue) are present within the epidermis (where they are called Langerhans cells) within other epithelia , and within lymphoid organs . A common feature of APCs is the presence of class II MHC molecules on their surfaces. CD4 T (helper) cells interact with complexes formed by peptides and class II MHC molecules on APCs whereas CD8 T (cytotoxic) cells interact with complexes of peptides with class I MHC molecules that can be presented by any nucleated cell .APCs being recognized by helper lymphocytes , are , thus essential for the triggering and development of a complex immune response.
The Langerhans cells of the epidermis constitute a very efficient system for trapping antigens that happen to enter the epidermis .these cells have many processes and upon capturing antigens .they retract the processes , move toward the dermis , and may enter a lymphatic vessel.
Types of immune response
The two types of immune responses are the (innate response and the adaptive response).the innate response ,which occurs through the action of neutrophils ,macrophages mast cells , and natural killer cells .is fast , nonspecific , and older from an evolutionary point of view, and it not produce memory cells ,the adaptive response , which depends on the initial recognition of antigens by B and T cells ,is much more complex , is slower and specific ,produces memory cells and is a more recent evolutionary development .few microorganisms are capable of penetrating intact skin , but many can enter sweat or sebaceous glands and hair follicles and establish themselves there sweat and sebaceous secretions by virtue of their acid PH and certain chemical substances (especially fatty acids) have antimicrobial properties that tend to eliminate pathogenic organisms lysozyme , an enzyme that dissolves some bacterial cell walls . is present on the skin and can help provide protection against some microorganism , lysozyme is also present in tears and in respiratory and cervical secretions .,the adaptive response can be antibody- mediated (humoral) cell –mediated (cellular) or both , an encounter with ,a microbial or viral agent usually elicits a complex variety of responses.



Lymphoid tissue
Lymphoid tissue is a type of connective tissue characterized by a rich supply of lymphocytes ,it exists free within the regular connective tissue or is surrounded by capsule , forming the lymphoid organs because lymphocytes have very little cytoplasm , lymphoid tissue stains dark blue in hematoxylin and eosinstained sections ,lymphoid tissues are basically made up of free cells as a result they typically have a rich network of reticular fibrils (made principally of type III collagen). That supports the cells. In most lymphoid organs the fibrils are produced by a fibroblastic cell called a reticular cell , whose many processes rest on the reticular fibrils, the thymus is an exception in so far as its cells are supported by a reticulum of epithelial cells of endodermic origin.