Immunoprophylaxis

IMMUNOPROPHYLAXIS

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Vaccines => active artificial immunity

Prophylactic immunoglobulins => passive artificial immunity

In evaluating immunological status, you must pay attention to avoid confunding immune responses toward the intended target from reactions caused by hypersensitivities towards other components (rarely antigen purification is absolute).

when cell-mediated immunity (CMI) is involved, assays usually are skin tests that look for type IV hypersensitivities (any test in which an antigen is applied to the skin in order to observe the response of the patient, described according to method of application, such as patch tests, scratch tests, and intradermal tests. Skin tests are used to determine prior exposure or immunity to an infectious disease (e.g., tuberculin test), to identify allergens producing allergic reactions, and to assess ability to mount a cellular immune response (using a battery of antigens that give positive tests in most normal individuals). Cutaneous or skin reaction / cutireaction / dermoreaction is a positive reaction indicating immunity or hypersensitivity in a skin test. It is recommended to create a negative control (e.g. in the controlateral half-body) in which the intended target only is inactivated : if positivity occurs also in the latter case, reaction is due to hypersensitivity against a component different from the intended one. False negativity may result from immune deficiencies .
when humoral immunity is involved, assays may consist of :

immunological demonstration of specific Abs
neutralization of intrinsic biological activity of the Ag (when present). Intrinsic biological activity may be confunded with type IV hypersensitivity against polluttants or against the Ag itself : obviously such hypersensitivities can occur both in presence and in absence of high titres of neutralizing Ab and so they are often misleading ! If the appearance of the positive reaction is identical to that of a type IV hypersensitivity, they can be distinguished only if they occur at different times (type IV hypersensitivity usually occurs after 24÷48 hours).

A positive or negative result doesn't give us information neither on past or present clinical statuses nor on effective protection against reinfection, but just on the presence or absence of some memory T lymphocytes or antibodies, respectively : anyway the presence of these lymphocytes is often considered equivalent to effective protection against a further challenge and immunoprophylaxis is not practiced. The latter consideration is not always true : e.g. natural infection with Mycobacterium tuberculosis , even when eradication of a primary infection by chemotherapy does occur, fails to protect against reinfection even if the existing memory lymphocytes are able to substain a type IV hypersensitivity against PPD.

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