Two general systems of immunity with specialized roles in defending against infection have been selected during evolution:

• Innate immunity, an ancient form of host defense, also called natural or congenital immunity, is an attribute of every living organism, present at birth, that is, before exposure to foreign agents and consisting of several nonspecific factors. Innate responses occur to the same extent as the infectious agent is encountered.

Not significantly modified after an encounter with non-self substances, it is void of both a fine-tuned discrimination of such substances and an increased activity following repeated encounters, thereby demonstrating that it does not possess memory.

The phagocyte cells (macrophages, neutrophils and monocytes) and alternative complement pathway, although void of specificity, are essential as primary elements of defense against a large number of infectious agents.

• Acquired immunity, also called adaptive immunity, becomes involved when the first level of defense fails to fully prevent infection, exemplifying a recent evolutive process, distinct by a particular specificity for offending antigens and by memory.

Unlike innate immunity, it is elicited or stimulated by exposures to intruders that escaped early elimination by the innate immune system, insofar as it is armed with a versatile discriminating capacity and potentiated by a successive encounter with such agents.

The major differences between these two types of immunity is: the effector mechanisms of innate immunity are activated immediately after infection and rapidly control the replication of infecting pathogens, so the infection is restrained until lymphocytes can accomplish their action.

It takes 3–5 days for a sufficient number of clones to be produced and differentiated into effector cells,which allows time for pathogens to damage the host.

The greatest difference is that acquired immunity, to compete with genetic variability of microorganisms, has lost the cardinal characteristic seen in innate immunity, that is, the ability to distinguish between potential pathogens and harmless substances. However, innate immunity may have an additional role in determining which antigens the acquired immune system responds to and the nature of that response.

Acquired Immunity

Acquired immunity involves T and B lymphocytes, antibodies and ILs, distinguished schematically into humoral and cell-mediated immunity (CMI), each equipped with various functions, partly different and partly overlapping. Acquired responses involve the proliferation of antigen-specific B and T cells,which occurs when the surface receptors of these cells bind to antigen.

Specialized cells, the antigen-presenting cells (APCs), display the antigen to lymphocytes and collaborate with them in the response to the antigen. Humoral-mediated immunity, above all responsible for primary defense against bacterial infections, is passively transferable by serum or plasma, being mediated by antibodies with a specific aptitude for reacting with the configurations responsible for its production.

Which is typical of B lymphocytes,which in humans differentiate in mammalian bursal equivalent tissue and acquire features of B cells from plasma cells with an endoplasmic reticulum (ER) characterized by an abundant RNA (ribonucleic acid). Specific antibodies are responsible for the reactions of immediate hypersensitivity, cytotoxicity, Arthus reaction and, by means of Fc receptors, phagocytosis.

CMI, as well as being active in the defense against viral infections, is implicated in some cases of autoimmune disease and is characteristically associated with effector–target cell interactions involved in antimicrobial immunity, rejection of allografts, immune surveillance and rejection of tumor cells.

This specific immunity is transferable by lymphoid cells and not by serum, where T lymphocytes play a fundamental role. Delayed-type hypersensitivity (DTH) is a typical CMI reaction, first discovered by Jenner.