Types of Specific Immunity

TYPES OF SPECIFIC IMMUNITY

Following are the various types of specific immunity that would be discussed briefly in the sections that follows :

1. Acquired Immunity

Acquired immunity usually results from the development of active or passive immunity, as opposed to either natural or innate immunity. 

In other words, acquired immunity invariably refers to the ‘protection’ an animal inherently develops against certain types of microorganisms or foreign substances. In reality, the acquired immu-nity gets developed in the course of an individual’s lifespan. Fig. 9.2 depicts the different types of acquired immunity in a summarized form.

2. Active Immunity

Active immunity refers to the specific immunity obtained from the development within the body of antibodies or sensitized T lymphocytes (T Cells) which critically neutralize or destroy the infective agent. It may eventually result from the immune response to an invading organism or from inoculation with a vaccine essentially containing a foreign antigen.

3. Cell-Mediated Immunity [or T-cell Mediated Immunity]

It has been duly observed that the regulatory and cytotoxic actions of T cells during the specific immune response is known as the cell-mediated immunity. However, the entire process essentially needs almost 36 hr to accomplish its full effect. It is also called as T cell mediated immunity.

Physiological Actions : Interestingly, unlike B cells, T cells invariably fail to recognize the so called foreign antigens on their own. A foreign antigen is duly recognized by a macrophage which engulfs it and displays part of the antigen on its surface next to a histocompatibility or ‘self’ antigen (macrophage processing). Finally, the presence of these two markers together with the secretion of a cytokine, interleukin-1 (IL-1) by macrophages and other antigen-presenting cells duly activates CD4+/ CD8 T cells (i.e., helper T cells), that categorically modulate the activities of other cells adequately involved in the immune response.

Thus, the CD4+T cells secrete interleukin-2 (IL-2), that stimulates the activity of natural killer cells (NK cells), cytotoxic T cells, and B cells ; and ultimately promotes the proliferation of CD+T cells in order that the invading pathogen may be destroyed or neutralized effectively. Besides, Gamma Interferon secreted by CD+T cells increases distinctly the macrophage cytotoxicity and antigen processing. However, the T-cell mediated immunity plays a significant and pivotal role in the rejec-tion of transplanted tissues and in ‘tests for allergens’ i.e., the delayed hypersensitivity reaction.

4. Congenital Immunity

The congenital immunity refers to the immunity critically present at birth. It may be either natural or acquired, the latter predominantly depends upon the antibodies solely received from the mother’s blood.

5. Herd Immunity

The herd immunity represents the immune protection duly accomplished via vaccination of a portion of a population, that may eventually minimise the spread of a disease by restricting the number of potential hosts for the respective pathogen.

6. Humoral Immunity [or B-cell Mediated Immunity]

Humoral immunity respresents the immunity duly mediated by antibodies in body fluids e.g., plasma or lymph. As these antibodies are adequately synthesized and subsequently secreted by B cells, that protect the body against the infection or the reinfection by common organisms, such as : streptococci and staphylococci, it is also known as B-cell mediated immunity. In reality, the B cells are stimulated by direct contact with a foreign antigen and differentiate into the plasma cells that yield antibodies against the antigen ; and the corresponding memory cells which enable the body to rapidly produce these antibodies if the same antigen appears at a later time.

It is, however, pertinent to state here that B cell differentiation is also stimulated duly by interleukin-2 (IL-2), secreted by the T4 cells, and by foreign antigens processed by macrophages.

7. Local Immunity

Local immunity is usually limited to a given area or tissue of the body.

8. Natural Immunity

Natural immunity refers to the immunity programmed in the DNA, and is also known as the genetic immunity. It has been observed that there are certain pathogens that fail to infect some species due to the fact that the cells are not exposed to appropriate environments, for instance : the ‘measles virus’ cannot reproduce in the canine cells ; and, therefore, dogs do have natural immunity to measles.

9. Passive Immunity

Passive immunity specifically refers to the immunity acquired by the introduction of preformed antibodies into an unprotected individual. It may take place either through injection or in utero from antibodies that usually pass from the mother to the foetus via the placenta. It can also be acquired by the newborn by ingesting the mother’s milk.