Specific immunity and the immune response

Specific immunity and the immune response

The diverse cells of the immune system are found in the blood and in organs and tissues throughout the body. A large number of chemical messengers called mediators are another important constituent of the immune system.

The dual nature of specific immunity

Immunization occurs when an individual is naturally or artificially exposed to an antigen, and the immune system is activated to produce humoral immunity and cellular immunity. This means that the effectors of specific immunity are found in both the humoral phase of the body fluids (e.g., blood serum) and among the white blood cells in the blood lymphoid organs. (Serum is the fluid portion of the blood that remains after the blood has been allowed to) These two effectors of immunity are physically and functionally different for instance, they resist different types of pathogens. Antibodies (soluble mediatory in body fluids) are more effective against pathogens found outside cells, and immune cells (cellular elements, specifically lymphocytes) are more effective against pathogens found inside cells. Thus humoral immunity defends the body primarily against bacteria, bacterial toxins, and viruses in body fluids. Cellular immunity defends te host from bacteria and viruses located within infected cells or phagocytic cells, as well as from fungi, protozoa, and other parasites, such as helminths. Cellular immunity is also responsible for rejection of transplanted tissues, and is important in defending the body against cancer in a process known as immune surveillance.

Characteristics of the specific immune response

The specific immune response has several characteristics that may be described as follows:

1. The specific immune system discrimination between “self” (not foreign) and “non-self” (foreign), ad responds only to materials foreign to the host. The cells of each individual have a unique array of self-marketing membrane molecules called histocompatibility proteins. They serve as cellular fingerprints that distinguish self from non-self.
2. The specific immune response is highly specific for the antigen to which the antibodies or immune cells will react with the greatest strength. This specificity underlies the system’s ability to discriminate between self and non-self.
3. The specific immune response is able to produce a greater response more quickly when there is a second exposure to the same foreign antigen. This is called immunologic memory, or the ama

Thus the four primary characteristics of the specific immune response can be summarized in these words: discrimination, specificity, anamnesis, and transferability by living cells.

The lymphocytes

The cells capable of responding to antigens are the lymphocytes, which are one of the classes of while blood cells. Lymphocytes are further divided into (1) thymus-derived lymphocytes, or T cells; (2) bone marrow-derived lymphocytes, or B cells; and (3) null cells.

Though the T and B lymphocytes cannot be distinguished on the basis of morphology, they have distinct membrane glycoproteins, carry out different functions, and respond to different activation stimuli, for example, different mitogens (substances that induce cell division, or mitosis).

Null cells are lymphocytes that lack distinguishing surface markers; they include the natural killer (NK) cells and antibody-dependent killer (K) lymphocytes. Otherwise, null cells are morphologically similar to T and B lymphocytes. Macrophages, monocytes, and polymorphonuclear leucocytes are also distinguishable by their surface markers and cellular functions.

The immune system

The lymphatic system consists of an interconnecting network of organs and tissues. A continuous traffic of cells moves along in the flow of blood and lymph that supplies this network. The circulatory and lymphatic systems are diagramed together to show how a pool of recirculating lymphocytes passes from the blood capillaries into the lymph nodes, spleen, and other tissues. The lymphocytes then circulate back to the blood via the major lymphatic channels such as the thoracic duct. Lymphocytes are the dominant cell type in most of the organs and tissues of the immune system. Thus such tissues and organs are described as lymphoid. They include all the body’s lymph nodes, the spleen, the adenoids and tonsils, small clusters of lymphoid tissue in the intestinal cell wall called Peyer’s patches, and the thymus.

Except for the thymus, the aforementioned tissues and organs make up the peripheral lymphoid system. Although the structure of the peripheral lymphoid system is centers of immune reactivity, they are described as peripheral because their development and function are absolutely dependent on cells generated in the thymus and in the bone marrow. For this reason, the thymus, and the bone marrow are considered the central lymphoid organs.

The hemopoietic, or hematopoietic (blood-forming), stem cells in the bone marrow are the ultimate origin of the erythrocytes (red cells) and of all leucocytes, including the lymphocytes. Many lymphocytes pass through the thymus and are processed by thymus hormones prior to release. The thymus is an organ whose sole function appears to be the differential of lymphocytes into T cells. The majority of the bone marrow-derived lymphocytes do not enter the thymus and are called B cells. Studies with chickens have revealed a central lymphoid organ called the bursa of Fabricius, which is responsible for including mature B-lymphocyte function, just as the thymus induces T-cell function. It is thought that the site in which human B cells mature (that is, the bursal equivalent) may be the fetal liver during embryonic development, followed in later life by the bone marrow. The B cells are destined to synthesize specific humoral antibody, and the T cells become primarily responsible for cellular immunity. The pathways of development of B cells and T cells.

How do the antigen-responding cells and the antigens find each other? Lymphocytes have surface receptors specific for different antigens. The function of the macrophages antigens and “show” them to the lymphocytes. In the lymph nodes and spleen, macrophages present antigen to the lymphocytes that are constantly circulating both within the organ and form the organ to the circulatory system and back again. Those lymphocytes which have receptors complementary to the specific antigen being presented will be selectively activated and then respond to the antigen. Therefore the antigen can be said to select and activate clones of those lymphocytes with the appropriate receptor. Such lymphocytes are called antigen binding cells. This is the essential concept of the clonal-selection theory of immunity. Seleted lymphocytes (whose receptors interact with a specific antigen) respond by undergoing mitosis and developing into a clone of cells expressing the same receptor specificity. Among the progeny of this clone will be plasma cells that secrete and synthesize antibodies specific for the stimulating antigen. Clonal selection also leads to the production of an enlarged pool of lymphocytes (called “memory cells”) that are sensitive to the specific antigen.

Humoral immunity

As indicated before, there are two broad categories of specific immune responses, namely, humoral and cell-mediated, or cellular, responses. B cells are the effectors of humoral immunity and are responsible for the synthesis of anti-bodies.

The antibody response

During the active immune response leading to specific immunity, the T lymphocytes and B lymphocytes, together with a type of macrophage, cooperate to generate the immune response. The macrophage processes and presents antigen in an optimal antigenic form to the T and B lymphocytes. The macrophage in this situation is called an antigen-presensting cells (APC). The T lymphocyte is activated to produce hormonelike factors (lymphokines) called interleukins, which promote B-lymphocyte growth and differentiation. B lymphocytes are specifically activated by antigen and the interleukins, producing antibodies as a result. Because the T-cell derived are required interleukins are required for B-cell activation, the T cells are often called helper T cells (th cells). Conversely, in some cases immune reponses are suppressed by certain T cells (Ts cells)-which act either by restricting the supply of essential growth factors or by producing molecules that inhibit lymphocyte activation. This suppression gives rise to a state of immune tolerance in which the animal fails to respond to the activating antigen.

Mediators of humoral immunity

The primary mediator of humoral immunity is the serum antibody. The first demonstration of this serum-borne immune factor was made by emil von behring in 1890, when he transferred serum from an immune donor animal to a susceptible recipient animal and then challenged the recipient with the virulent pathogen that had made the donor immune. The animal receiving the immune serum resisted the pathogen because the transfer of the performed antibodies form the donor had conferred temporary immunity upon the recipient. Since the recipient did not actively produced the antibodies but just passively received them as a gift from the donor, this type of immunity is called passive immunity. This is in contrast to the introduction of active immunity by exposure of the recipient to either the natural disease or a vaccine that induces the recipient’s own immune system to produce the antibodies. Thus specific immunity may be acquired either passively or actively, and by natural or artificial means. Serum contains about 8 percent protein, in addition to salts and other components. The antibodies are found among the globular proteins and are often referred to as immunoglobulinas (Ig’s). A common research method used to study immunoglobulinas utilizes an electric field to separate the serum proteins. This method is called electrophoresis. All proteins have many positively charged free amino groups (-NH₃) and negatively charged free carboxyl groups (-CO₂). But if protein has unique net charged and molecular weight, which determine its relative mobility in an electric field. Positively charged proteins will migrate toward the cathode (negative pole), and negatively charged proteins will travel toward the anode (positive pole). The serum is separated into albumin and several groups of globulins labelled alpha (), beta (), and gamma (). The gamma region contains the most globulins.

References

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