When an antigen is initially encountered, the cells of the immune system recognize the antigen as nonself and elicit an immune response or become tolerant of it, depending on the circumstances. An immune reaction can take the form of cell-mediated immunity (immunity dependent on T cells and macrophages) or may involve the production of antibodies (B lymphocytes and plasma cells) directed against the antigen.

Production of antibodies is induced when the host’s lymphocytes come into contact with a foreign antigenic sub stance that binds to its receptor. This triggers activation and proliferation, or clonal selection. Clonal expansion of lymphocytes in response to infection is necessary for an effective immune response (Fig. 1). However, it requires 3 to 5 days for a sufficient number of clones to be produced and to differentiate into antibody-producing cells. This allows time for most pathogens to damage host tissues and cells.

Fig1. Primary and secondary antibody response. (Adapted from Turgeon ML: Fundamentals of immunohematology, ed 2, Baltimore, 1995, Williams & Wilkins.)

Whether a cell-mediated response or an antibody response takes place depends on how the antigen is presented to the lymphocytes; many immune reactions display both types of responses. The antigenicity of a foreign substance is also related to the route of entry. Intravenous and intraperitoneal routes are stronger stimuli than subcutaneous and intramuscular routes.

Subsequent exposure to the same antigen produces a memory response, or anamnestic response, and reflects the outcome of the initial challenge. In the case of antibody production, the quantity of IgM-IgG varies.

Primary Antibody Response

 Although the duration and levels of antibody (titer) depend on the characteristics of the antigen and the individual, an IgM antibody response proceeds in the following four phases after a foreign antigen challenge (see Fig. 1):

1. Lag phase—no antibody is detectable.

 2. Log phase—the antibody titer increases logarithmically.

 3. Plateau phase—the antibody titer stabilizes.

 4. Decline phase—the antibody is catabolized.

Secondary (Anamnestic) Response

 Subsequent exposure to the same antigenic stimulus produces an antibody response that exhibits the same four phases as the primary response (see Fig. 1). Repeated exposure to an anti gen can occur many years after the initial exposure, but clones of memory cells will be stimulated to proliferate, with subsequent production of antibody by the individual. An anamnestic response differs from a primary response as follows:

 1. Time. A secondary response has a shorter lag phase, longer plateau, and more gradual decline.

2. Type of antibody. IgM-type antibodies are the principal class formed in the primary response. Although some IgM antibody is formed in a secondary response, the IgG class is the predominant type formed.

 3. Antibody titer. In a secondary response, antibody levels attain a higher titer. The plateau levels in a secondary response are typically 10-fold or greater than the plateau levels in the primary response.

An example of an anamnestic response can be observed in hemolytic disease, when an Rh-negative mother is pregnant with an Rh-positive baby (see Chapter 26). During the moth er’s first exposure, the Rh-positive RBCs of the fetus leak into the maternal circulation and elicit a primary response. Subsequent pregnancies with an Rh-positive fetus will elicit a secondary (anamnestic) response.

Vaccination is the application of primary and second responses. Humans can become immune to microbial antigens through artificial and natural exposure. A vaccine is designed to provide artificially acquired active immunity to a specific disease (e.g., hepatitis B). Booster vaccine (repeated antigen exposure) allows for an anamnestic response, with an increase in antibody titer and clones of memory cells (see Chapter 16).

مواضيع ذات صلة

الحساسية الدوائية نحو العوامل المخصرة للوصل العَصَبِي العَضَلِي

آليات حساسية السلفوناميدات ومظاهرها

Antigen - Antibody Interaction: Specificity and Cross - Reactivity

Tolerance to Commensal Microbes and Fetal Antigens

B Lymphocyte Tolerance

Peripheral T Lymphocyte Tolerance

Immune Memory

B Cells

T Cells

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الحساسية نحو البنسلين

الفرق بين فَرْطُ التحسس والحساسية الدوائية