eChapters

Immunity is defined as the resistance to infection. This is carried out by the process of identification and removal of non-self material. In contrast to this, our body has to maintain immune tolerance to our own cells and tissues and develop the ability to distinguish these self-antigensfrom non-self (foreign) antigens. Tolerance means unresponsiveness of the immune system to an antigen. Immune tolerance is the process by which the immune system does not destroy an antigen. Tolerance can be either natural or induced.

One of the greatest achievements in science is the development of vaccines. Vaccines have been revolutionary for the prevention of infectious diseases. They have prevented a large number of deaths caused, every year, by dreadful diseases like smallpox, polio, hepatitis etc. Diseases like smallpox and polio have been successfully eradicated by vaccination. Normally, when the body encounters a microbe, both humoral and cytotoxic immune responses are induced to neutralize its effect. In addition to eliminate the invader, body forms memory cells which help the body to protect itself from the same pathogen, in future. Vaccines not only function to kill the pathogens but they also stimulate the body to form memory cells which provide immunity against a specific pathogen

Infection refers to the invasion of harmful microorganisms into the body and causing disease. The most important function of the immune system is to protect the body of the host against various viruses, bacteria, fungi and parasites. The grounds of immune surveillance stand on the basis of innate and specific immunity that build up against the evasive mechanisms of the disease causing organisms. Infectious disease is result of a complex interaction between  the host and the microbes. Evasions of protective immunity by the parasites critically determine the survival and pathogenicity of the microbes. The invasion of the body by harmful microorganisms is called infection. Infections are dangerous in any part of the body. Microorganisms spread vary easily into the blood stream and cause blood poisoning which is called septicemia.

The immune system has been designed to provide protection to the body from various diseases. But the same system which is aimed to provide protection may sometimes become harmful to the body and may even life-threatening. There are situations when immune response against harmless and inert antigens is exaggerated and actually causes diseases. Such a condition is called hypersensitivity or commonly as allergy. Though the term ‘hypersensitivity’ indicates an increased response but the immune response is not always heightened, instead it may be an inappropriate response to an antigen.

Cytokines are the chemical messengers that are involved in interactions between the cells of innate and adaptive immune responses. They are secreted by leukocytes and many other cells in the body. They are low molecular weight proteins or glycoproteins which are produced in response to various kinds of stimuli. Cytokines have important role in immune and inflammatory responses.  Their functions are not limited to immune system only.They also stimulate the development of hematopoietic cells and also act as therapeutic agents.

Major Histocompatibility Complex (MHC) is a closely linked cluster of genes which have codes for cell surface proteins known as  MHC molecules. These molecules have very important role in presentation of processed antigens to the T cells in initiation of an immune response as T cells recognize an antigen only when it is presented along with MHC molecule. These are highly polymorphic and are the major barrier to organ/tissue transplantation. All vertebrates have MHC. MHC differs among members of a species and are thus called alloantigens. MHC participates in the development of both humoral and cell-mediated immune responses.

The complement system is a group of about 20 heat labile plasma proteins which constitute nearly 10% of total serum proteins. These are important in activation of phagocytes, control of inflammation and cytolysis. They form one of the major immune defense systems of the body. The complement system plays important role both in innate and adaptive immune system. The main functions of complement are chemotaxis; opsonization and cell activation; immune clearance of immune complexes from circulation; lysis of target cells; triggering and amplification of inflammatory reactions and development of antibody responses. Evolutionary, the system is very ancient and is present even in invertebrate organisms like starfish and worms.

Cell mediated immune response performs different functions in protecting the immune system. When antigens are destroyed by cells, without producing antibodies, the immunity is called cell-mediated immunity (CMI) or cellular immunity. In this immunity T cells play a major role. The principal role of cell mediated immunity is to detect and eliminate the cells that have intracellular pathogens. When the antigen remains inside the cell the antibody cannot handle them. The intracellular pathogens are controlled by cell- mediated immunity. CMI is invited by the recognition of antigens by T cells. T cell lymphocytes recognize protein antigens of intracellular microbes that are displayed on the cell surface of infected cells. It is also important for elimination of cells which express foreign MHC molecules. Antigen specific and nonspecific cells can contribute to the cell-mediated immune response. The activities of both specific and non-specific components usually depend on effective local concentration of cytokines.

The immune system protects the body from harmful substances by recognizing and responding to antigens. Antigens are the protein molecules which are present on bacteria, virus, fungi or some other microorganisms.Some toxins can also be antigens. These antigens are recognized by immune system. The destruction of antigens by producing antibody is called antibody-mediated immune response. As antibodies are present in the body fluids i.e. humors,this type of response is called humoral immune response.

All blood cells arise from a basic cell called the hematopoietic stem cell. Stem cells are undifferentiated embryonic cells.They are self renewing and their population by cell division. Stem cells have the ability to develop into any type of blood cell. The formation of blood cells begins in the embryonic yolk sac during the first week of development. Every mature cell is derived from the same type of stem cell. In contrast to an unipotent cell which differentiates into a single cell type, a hematopoietic stem cell is multipotent or pluripotent. In the mammalian embryo, the blood corpuscles are normally formed in yolk sac, liver, spleen and thymus gland.

Antigen-antibody interaction is very specific one in which an antibody recognizes an antigen through complementarities of shapes on paratope and epitope.  It is like lock and key, as occurs in case of enzyme-substrate reaction. It does not involve covalent linkage rather a spatial complementarity occurs between antigen and antibody. For example, when antibodies, raised against m-amino benzene sulphonate, are tested for their ability to bind with ortho-, meta- and para-isomers of the hapten, the antibodies show highest strength with meta than ortho- isomer.  Antibodies show poor reactivity with para isomer. This is because though chemically all these molecules are same but structurally they are different.

Antibodies are a group of glycoproteins having a globular compact structure. They are known as immunoglobulins. On the basis of their experiments, Tiselius and Kabat, in 1939, demonstrated that most antibodies are found in the g globulin fraction of serum proteins. When serum from rabbit, immunized with ova albumin, was subjected to electrophoresis, it showed a peak for g globulin (Fig. 4.1). Studies have revealed that the major part of IgG is found in the g globulin fraction of the serum proteins. Other fractions, a and b, too, have other immunoglobulins along with small part of IgG.

The main purpose of the immune system is to protect the body from foreign substances, diseases or infections. When the body is exposed to a microbe or a foreign substance (antigen), the body makes some response which is called immune response. The purpose of immune response is to recognize, neutralize and eliminate the antigens from the body. Generally, the term immunogen is also used for antigen. Both, antigen and immunogen are used as synonyms but specifically, they are different from each other.

Like any other organ system in the body, immune system comprises of various organs and cells. The major component of the immune system is the lymphocytes which constitute lymphoid system. The lymphoid organs are classified into two categories: primary and secondary lymphoid organs.

Primary lymphoid organs: These are the organs where B and T lymphocytes undergo maturation. Bone marrow and thymus are the primary lymphoid organs. These are the organs where lymphocytes differentiate, proliferate, get selected and mature into functional cells.

Everyone on this earth wishes to have a healthy life free from all ailments, stress and strain. But it is a common observation that some of us are more susceptible to diseases and fall victim of many infections while others have the capacity to resist many such infections. They are not so much susceptible to diseases. Is there anything which distinguishes them from others? An organism is made up of organs and organs systems. Like many other organ systems, in the body there is a system which is concerned with the protection of the body from infections.

Reducing insecticide application to the lowest level is the motto of IPM in vegetables. Monitoring for the initial build up and timely mechanical removal of the first population is a must in vegetables. Always remove the first affected plants, by diseases like mosaic, and destroy them by burying or burning. Remove insect eggs, insect affected plant parts and adults and immature stages of insects, and destroy them every day. Do not throw them in the field. They will either come back, or pupate and attack later. During nursery stage, use wood ash, cow dung solution, and plant extracts to deter all pests. Use only low residual insecticides like Dichlorvos 1ml/l, Malathion 2ml/l, Endosulfan 2ml/l or Quinalphos 2ml/l, in spots, at the beginning of attack itself.

Fruit borer: Helicoverpa armigera: Noctuidae

This is a medium sized moth yellowish brown in colour with a wing

span of about 3 cm and a body length up to 1.4 cm. There is a dark speck and a dark area on the outer margin of each forewing. The forewings also have greyish wavy lines and black spots of different sizes on the upper side.

Sciarid(mushroom flies): Bradysia paupera, Lycoriella auripila: Sciaridae

The larvae feed on the mycelial attachments and restrict the growth of the spawn. The maggots also feed on the developing mushrooms or pinheads and thus cause loss in yield. They also bore the developing mushrooms and render them hollow.

Fruit flies: Bactrocera cucurbitae:Tephritidae

This pest is more serious in bitter gourd, snake gourd and to a certain extent on ridge gourd. Other cucurbits are attacked to a lesser level. Damage by this pest causes losing the fruit quality, rotting and fruit drop. The adults lay many eggs inside the fruit sing their big ovipositor that hatch out into worm like apodous maggots. Feeding by the maggots on the pulp of fruits result in rotting, yellowing and fruit drop. The dropped fruits allow pupation by the maggots in the soil from where they emerge out as adults and start egg laying on fresh frits.

The cowpea aphid: Aphis craccivora: Aphididae

Hosts
 
Groundnut, red gram, peas, beans, safflower, lab-lab niger

Symptoms of damage
 
Both nymphs and adults cause the damage by sucking the plant sap from shoot tips, pods and leaves.  Infested pods become malformed and withered.  Severe infestation may result in complete drying of affected pods.  They also act as vector of virus diseases in pea.