| THE SPECIFIC IMMUNE RESPONSE
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| The specificity of the immune response is achieved through special receptors that recognize antigen
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| The specific immune response is brought into play if the nonspecific defenses are unsuccessful, e.g. due to the persistence of the triggering agent. The specific response focuses on the individual antigen by using specific receptors that can be viewed as being individually tailored to the stimulating element. The antigen and receptors can be considered to have a 'hand-in-glove' relationship, fitting together in a unique three-dimensional manner. A prerequisite for such response is the ability of the system to distinguish between self and nonself. This is achieved by thymic education and self-tolerance.
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A young man was brought into the emergency room in a state of shock with stridor and widespread urticaria. A companion told the admitting medic that the patient had just been stung by a bee. A diagnosis of anaphylaxis was made. An intramuscular injection of epinephrine was given promptly and also treatment with intravenous antihistamine, corticosteroid, and cardiorespiratory support. The man recovered. |
| Comment. While the physiologic role of the IgE response is considered to be protection against parasite infestation, this response is seen to be subverted in those who experience atopic diseases and anaphylaxis. The atopic diseases include allergic rhinitis, allergic conjunctivitis and asthma. The major fraction of IgE is bound via receptors to mast cells in the tissues. When antigen binds and crosslinks its specific IgE on the mast cells, it triggers the degranulation of the cells and release of preformed mediators (principally histamine), as well as the synthesis of other mediators, including arachidonic acid metabolites. The clinical effects depend on the location and/or extent of mast cell degranulation. When localized to one site, such as the nasal or bronchial mucosa, it usually gives rise to only localized reactions in the form of allergic rhinitis and asthma, respectively. If the degree of sensitization with the antigen-specific IgE and/or the antigenic burden is much greater, systemic degranulation can occur, with consequent anaphylactic shock. This is primarily due to the effects of the released mediators on vessels and vascular integrity. Significant vasodilation takes place, reducing the blood pressure. This is accompanied by large increases in vessel wall permeability, leading to shifts of fluid from the intravascular to the extracellular compartment and substantial swelling of any tissues with the capacity, which particularly affects the skin and other loose connective tissue such as those in the larynx. Smooth muscle spasm also occurs, leading to bronchoconstriction and contraction of the gut wall, with consequent marked respiratory difficulty and wheezing. These features are accompanied by increased secretory activity of seromucous glands in the respiratory and gastrointestinal tract as well as itching of the skin. |
| When a specific immune response is initiated, relatively few components are likely to be available that could react specifically with any chosen antigenic substance. There is a delay or lag period while these components increase to a level at which they can ensure elimination of the antigen, or at least reduce it to such a level that would be manageable by the nonspecific immune response. This delay can have disastrous consequences for the organism and its survival.
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| In addition to generating specificity, the specific immune response also employs a mechanism to remember a specific encounter so that if the same foreign or nonself substance is encountered again, it can be dealt with more quickly and
effectively. Thus, in comparison to the nonspecific immune response, the specific immune response shows both the specificity for the foreign or nonself substance - the memory of the event.
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Similarly to the nonspecific response, specific immune response is mediated by cellular and humoral elements. The cells primarily responsible are the lymphocytes that reside in the lymphoid tissues; there are two major types:
- T cells which are responsible for cellular specific immunity,
- B cells which are responsible for humoral specific immunity.
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| The terms 'cellular' and 'humoral' have a slightly different meaning than when used in the context of nonspecific response. 'Cellular' refers to those aspects which are driven by T cells and 'humoral' to the aspects mediated by antibodies (which are the products of the other major lymphocyte type: the B cell).
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