Understanding the Specific Immune Response: Mechanisms and Key Players
1.2) Mechanisms of the Specific Immune Response
When non-specific defenses prove insufficient, the body activates its specific immune response within a few days. This intricate system distinguishes between “self” and “foreign” molecules, targeting and eliminating foreign invaders.
Types of Leukocytes in Human Blood
- Basophils: Involved in allergic reactions and release heparin in tissues.
- Lymphocytes: Key players in the specific immune response.
- Monocytes: Phagocytose bacteria, dead cells, and organic debris. They mature into macrophages (large predatory cells) after 24 hours in the blood.
There are two main types of specific immunity: antibody-mediated immunity (humoral response) and cell-mediated immunity (cellular response).
The Immune System
Both types of immunity are carried out by lymphocytes, which originate from stem cells in the bone marrow.
Primary Lymphoid Organs (Maturation Sites)
- Thymus: Produces T-lymphocytes.
- Bone Marrow: Produces B-lymphocytes.
After maturation, lymphocytes travel through the blood and lymph to secondary lymphoid organs. B-lymphocytes possess immunoglobulins that detect soluble antigens, while T-lymphocytes have receptors that recognize antigen fragments on the surface of other cells.
Antigens
Antigens are exogenous (foreign to the body), immunogenic (capable of inducing antibody formation), and react specifically with these antibodies. The regions responsible for antibody stimulation and interaction are called antigenic determinants.
Immunoglobulins (Antibodies)
Immunoglobulins (Ig) are produced in response to specific antigens and are found in blood, lymph, and other bodily secretions. Each antibody consists of two identical heavy chains (H) and two identical light chains (L).
The variable region, responsible for antigen binding, is called the binding domain. The constant region, known as the effector domain, activates phagocytes and the complement system.
Important Immunoglobulins in Humans
- IgA: Abundant in mucus of the nose, respiratory and gastrointestinal tracts, tears, saliva, and vaginal secretions.
- IgE: Mediators in allergic reactions.
- IgM: First to be synthesized in response to an antigen, effective against bacteria.
- IgG: Most abundant, replace IgM during the immune response. IgG is the most prevalent group in humans.
Antigen-Antibody Reactions
Antibodies recognize pathogens by binding to their surface antigens. This complex formation triggers various defensive reactions:
- Agglutination of antigen-antibody complexes.
- Blocking pathogen or toxin activity.
- Stimulation of phagocytes.
- Activation of the complement system by IgG and IgM antibodies.
Humoral Response (Antibody-Mediated Immunity)
This response involves antibody synthesis by B-lymphocytes. When a foreign antigen enters the body, it encounters a B-cell with a matching antibody. This interaction stimulates the B-cell to divide and differentiate into two classes:
a) Plasma Cells
Mature B-cells that develop extensive rough endoplasmic reticulum for synthesizing and exporting large quantities of antibodies. Plasma cells remain in lymph nodes, releasing antibodies that travel to the infected area via the lymph.
b) Memory Cells
Remain in circulation, producing small amounts of antibody long after the infection.
The Complement System
This system comprises 18 proteins that assist antibodies in fighting infection. These proteins bind to pathogens, facilitating their uptake by macrophages or directly destroying them.
Cellular Response (Cell-Mediated Immunity)
This response relies on the activity of T-lymphocytes and macrophages. When an antigen invades, macrophages engulf and digest it, fragmenting it into peptides. These fragments are presented on the macrophage’s surface bound to MHC proteins, which are recognized by T-cells.
T-Cell Activation and Differentiation
T-cells possess specialized receptors that recognize and bind to antigen fragments, leading to their activation.
a) CD8 T Cells (Killer Cells)
Secrete cytotoxins or release cytokines, also known as cytotoxic T-lymphocytes.
b) CD4 T Lymphocytes (Helper Cells)
Release lymphokines, promoting the proliferation of CD8 T-cells and B-cells, and enhancing inflammation. These cells work together in a highly interactive system.