Introduction to Immunology

  

Immunology is the branch of medical science which deals with antigen–antibody reaction. Immunity refers to all mechanism used by the body as protection against environmental agents that are foreign to the body. These agents maybe microorganisms or their products: foods, chemicals, drugs, pollen or animal hair or dander.

Types of immunity

1. Innate immunity is conferred by all those elements with which an individual is born and which are always present and available at very short notice to protect the individual from challenges by foreign invaders.

a. These elements include body surfaces and internal components, such as the skin, the mucous membranes and the cough reflex, which present effective barriers to environmental agents.

b. Chemical interferences such as pH and secreted fatty acids constitute effective barriers against invasion by many microorganisms.

c. Internal elements include fever, interferons, beta–lysine, enzyme lysozyme, polyamines, kinins, phagocytic cells (i.e., granulocytes, macrophages and microglial cells of the CNS).

2. Acquired immunity is a process by which individual acquires the immunity to withstand and resist a subsequent attack by, or exposure to, the same offending agent.

Types of acquired immunity:

a. Active immunization refers to immunization of an individual by administration of an antigen.

b. Passive immunization refers to immunization through the transfer of specific antibody from an immunized individual to a non–immunized individual.

c. Adoptive transfer (immunization) refers to the transfer of immunity by the transfer of immune cells.

Characteristics of the immune response:

The acquired immune response has several generalized features that characterize it and serve to distinguish it from other physiologic systems, such as circulation, respiration and reproduction. These features are as follows:

1. Specificity

The ability to discriminate among different molecular entities presented to it and to respond only to those uniquely required rather than making a random, undifferentiated response.

2. Adaptiveness

This is the ability to respond to previously unseen molecule that may never have existed before on earth.

3. Discrimination between “self” and “non–self”

This is the ability to recognize and respond to those molecules that are “self.” This distinction and the recognition of antigen, is conferred by specialized cells, namely, lymphocytes, which bear on their surface receptors specific for antigen.

Moreover, as different lymphocytes bear different receptors specific for different antigens, each cell also bears identical receptors specific for an identical antigen or a portion of the antigens.

4. Memory

This is the ability to recall previous contact with a foreign molecule and respond to it in a learned manner through the help of the nervous system. The term used to describe immunologic memory is anamnestic response.

Branches of acquired immunity

1. Humoral immunity

Primarily mediated by serum antibodies which are the proteins secreted by the B cell compartment of the immune response. B cells are initially activated to secrete antibodies after the binding of antigens to specific membrane immunoglobulin molecules (B cell receptor/BCR), which are expressed by these cells.

Another important element involved in humoral immunity is the complement system. The reaction between antigen and antibody serves to activate this system, which consist of a series of serum enzymes, the end result of which is lysis of the target or enhanced phagocytosis (ingestion of the antigen) by phagocytic cells.

The activation of complement also results in the recruitment of highly phagocytic polymorphonuclear cells, which constitute part of the innate immune system. These activities maximize effective response made by the humoral arm of immunity against invading agents.

2. Cell mediated immunity

This is primarily mediated by T–lymphocytes. Each T cell bearing many identical antigen receptors called T cell receptors (TCR), circulates directly to the site of antigen and performs its function when interacting with antigen. This is also referred to as Delayed type hypersensitivity.

Cells involved in Acquired Immune Response

1. T cell – primarily produced in the thymus gland

2. B cell – primarily produced in the bone marrow

3. Antigen presenting cells (APC) such as macrophages and dendritic cells – function to process and present the antigen to the specific receptors (T cell receptors/TCR) on T – lymphocytes.

Two types of APC molecules:

a. MHC Class I – presented to and participates in activation of cytotoxic T cells

b. MHC Class II – presented to and participates in activation of helper T cells

·       MHC (Major Histocompatibility Complex)

4. Cytokines – soluble substances secreted by cells which have a variety of effects on other cells.

The Clonal Selection Theory

The specificity of the immune response is based on the ability of its components (namely, antigen–specific T and B lymphocytes to recognize particular foreign molecules (antigens) and respond to them in order to eliminate them. Inherent in this theory is the need to clonally delete lymphocytes that maybe self or autoreactive. If such a mechanism were absent, autoimmune responses might occur routinely.

1. T and B lymphocytes of myriad specificities before there are any contact with the foreign antigen.

2. The lymphocytes participating in the immune response have antigen – specific receptors on their surface membranes.

a. In the case of B lymphocytes, the receptors are molecules (antibodies) bearing the same specificity as the antibody that the cell will subsequently produce and secrete.

b. T cell products are not the same as their surface receptors but are other protein molecules called cytokines that participate in elimination of the antigen by regulating the many cells needed to mount an effective immune response.

3. Each lymphocyte carries on its surface receptors molecules of only a single specificity.

4. Immunocompetent lymphocytes combine with the foreign antigen, or a portion of it, termed epitope, by virtue of their surface receptors. They are stimulated under appropriate conditions to proliferate and differentiate into clones of cells with the corresponding identical receptors to the particular portion of the antigen, termed antigenic determinant or epitope.

a. With B cell clones, this will lead to the synthesis of antibodies having precisely the same specificity. Collectively, the clonally secreted antibodies constitute the polyclonal antiserum, which is capable of interacting with the multiple epitopes expressed by the antigen. Several different clones of B cells will be stimulated to produce antibody, whose sum – total is an antigen – specific antiserum that comprises antibodies of differing specificities.

b. T cells will be similarly selected by appropriate antigen or a portion. Each selected T cell will be activated to divide and produce clones of the same specificity. Thus, the clonal response to the antigen will be amplified; the cells will release various cytokines and subsequent exposure to the same antigen would now result in the activation of many cells or clones of that specificity.

5. Circulating self–antigens that reach the developing lymphoid system before some undesignated maturational step will serve to shut off those cells that recognize it specifically and no subsequent immune response will be induced.