Adaptive immunity

Adaptive immunity is the body’s ability to recognize and mount an effective immune response against invaders whether bacteria, protozoa, or viruses. Adaptive immunity relies on antibodies that are produced by differentiation of B cells, and which recognize antigens. Antigens are molecules that trigger an immune response and maybe a portion of a cell, a whole organism, that the body recognizes as deems foreign.

• Specificity – This refers to the ability of immune cells to recognize and act on one particular shaped-molecule and not others. The role of specificity in antibody structure and the immune response is that a specific epitope on pathogen stimulates the B cell receptor, the B cell responds by proliferating and dividing and secretes immunoglobulins against the pathogen.
• Inducibility – THis refers to the ability of adaptive immunity to only be activated by specific pathogens. This ensures the immune system does not attack host cells.
• Clonality – The immune cells have the ability to multiply to several generations of the same cells which are nearly identical.
• Memory – The adaptive immunity keeps memory after attacking a specific pathogen so as to respond faster if attacked by the same pathogen in the future.

The two basic types of adaptive immunity are the humoral response and cell-mediated response. The cell-mediated response utilizes T-cells while the humoral-mediated response utilizes the B-cells.

Antigens

The antigenic determinant is an epitope that is characterized by a three-dimensional shape, which is recognized by the immune cells. Antigens are either exogenous or endogenous. Exogenous antigens are produced mainly by organisms like bacteria which multiply outside the body. Endogenous antigens are produced by microorganisms that multiply within the body such as multiplication of virus particles after infection. Autoantigens are a type of antigens that are found on the surface of the normal body cells.

Lymph and lymphatic system

Lymph is a colorless liquid that leaks out of the blood vessels into the intracellular space which contains materials such as degraded proteins and toxins. Lymph cells arise from the red bone marrow and one most important type of lymph cell in the immune system is the lymphocyte which is a granulocyte,

In the lymph system, the function of lymph nodes is to house the lymphocytes waiting for foreign invasion. The body contains numerous lymph nodes, with the main ones being the groin located at the groin, the cervical lymph nodes, axillary, and others in the abdominal region. When a foreign substance is introduced in the system, it is neutralized in these regions and excreted through a one-way-system.

The spleen and it’s role

Spleen is part of the lymphatic system and the immune system as well. This organ is similar in structure as the lymph nodes. The main functions of the spleen in the lymphatic system are to filter and screen microorganisms such as bacteria, viruses, and toxins from the blood and acting as the primary site for the storage of iron and platelets among other important components of blood.

What are Peyer’s Patches?

Peyer’s patches are defined as a discrete portion of lymphoid tissue in the small intestine that is mucous-associated. They are considered a crucial part of the immune system by monitoring populations of bacteria in the intestines and ensuring less growth of the pathogenic bacteria.

The B lymphocytes

The B lymphocytes are specific types of lymph cells formed and mature in the red bone marrow. They are localized in the spleen upon maturation, and also in the lymph nodes, red bone marrow, and Peyer’s patches. They circulate in the blood and respond to antigens when triggered by the other innate cells. The main function of B cells in the immune system is to secrete antibodies or immunoglobulins.

Structure of a lymph node consists of three parts: The central region which has passages filtering the lymph, the medulla which recognizes any foreign particle in the lymph, and the cortex which is the outermost part that is responsible for B-cell replication.

Mucus-Associated lymphoid tissue (MALT)

Mucous membranes of the appendix, urinary bladder, mammary glands, Peyer’s patches, and respiratory tract. The main function of MALT is to initiate a response against all foreign particles or pathogens recognized at the lymphoid associated sites.

Immunoglobulins

Antibodies are antigen-binding sites that are part of humoral immunity. There are several types of antibodies that are classified into classes depending on the structure.

• Immunoglobulin G – This antibody lasts the longest period in the blood, is the smallest, and has a single y-shaped monomer. One exception of the Immunoglobulin G which is not found in the other classes of antibodies is that they can easily penetrate the placenta to protect the fetus against possible infection.
• Immunoglobulin A – This antibody consists of two monomers connected by J chain, and is also called secretory IgA fighting antigens by agglutination, and neutralization.
• Immunoglobulin M – This has subunits that combine to form a 5-unit polymer. This antibody is relatively larger than the others and is due to this reason that it plays a critical role in complement activation.
• Immunoglobulin E – This antibody is found in low concentrations in the serum and is mainly used as a signal molecule. It helps to trigger the release of chemicals by the eosinophils when it recognizes a pathogen. Thus, rather than neutralizing the pathogen on its own, it presents the pathogen to the eosinophils.
• Immunoglobulin D – Less is known about this antibody.

Immune cells

Plasma cells secrete immunoglobulins, which are Y-shaped. There is the Fragment antigen-binding (fab), the Fragment crystallization (Fc), and the constant region. The fab is the angle between the arm and stem, the Fc is the tail region of the structure which binds cell surface receptors and the constant region determines which mechanism the antibody will use to destroy the pathogen. In regards to the constant region of the antibody, there are five major classes: IgM, IgG, Iga, IgD, IgE.

T lymphocytes are formed by the red bone marrow, matures in the thymus, and circulates within the blood and lymph. They are also found in the Peyer’s patches and spleen. They directly attack pathogens. Types of T lymphocytes include the cytotoxic T cells, Helper T cells. Cytotoxic T cells directly attack cells that are infected with the pathogens especially viruses. Infected cells present different epitopes on their cell surface which can easily be identified by the cytotoxic T cells by the help of MHC I and II.

Memory B cells have B cell receptors (BCR’s) located on their surface, which are complementary to specific epitopes. These cells may remain in the body for more than 20 years, and can effectively trigger the release of antibodies once the same epitope is recognized in the future.

Activated T cells that remain active for a long period within the body are called memory T cells.

The functions of antibodies

1. Activating the complement system
2. Neutralizing the antigen when the antibody binds to the most critical part of the antigen making it unable to harm the body or bind to cells.
3. Opsonization of the antigen by stimulating phagocytes and macrophages
4. Killing by oxidation through hydrogen peroxide and ozone among others.
5. Agglutination whereby many antibodies clump together hindering the activity of the antigen and exposing it to the immune cells
6. Cellular cytotoxicity through the use of natural killers which lyse antigens by producing perforins and granzyme.

The B cells can be deleted from the body through a process called clonal deletion. This serves to ensure B cells are not reactive against host cells.

The steps involved in clonal deletion

Generation of new B cells in the red bone marrow. Formation of a distinct B-cell receptor on each cell. Cells with complimentary BCR to autoantigens react causing apoptosis deleting the entire clone of similar B cells. B cells possessing unique BCR and which do not react to autoantigens are released into the blood.

Types of immunity

Naturally acquired active immunity refers to the immune response that is triggered on a daily basis when a pathogen invades the body.

Naturally acquired passive immunity is acquired through birth (from the mother to the fetus).

Artificially acquired active immunity is acquired when the antigen is transferred directly into the organisms, mostly when a live vaccine is used.

Artificially acquired passive immunity occurs when someone is injected with antibodies against a particular antigen.

Term	Definition
Adaptive Immunity	Body’s ability to recognize and mount an effective immune response against invaders
Specificity	Ability of immune cells to recognize and act on a specific shaped-molecule
Inducibility	Ability of adaptive immunity to be activated only by specific pathogens
Clonality	Ability of immune cells to multiply and produce generations of nearly identical cells
Memory	Ability of adaptive immunity to retain memory of attacking a specific pathogen
Humoral Response	Type of adaptive immunity mediated by B cells and antibodies
Cell-Mediated Response	Type of adaptive immunity mediated by T cells
Antigens	Molecules that trigger an immune response and can be recognized as foreign
Lymph	Colorless liquid containing degraded proteins and toxins
Lymphatic System	System consisting of lymph nodes, spleen, and other organs involved in lymph production
Spleen	Organ in the lymphatic system that filters microorganisms from the blood
Peyer’s Patches	Discrete portions of lymphoid tissue in the small intestine that monitor bacterial populations
B Lymphocytes	Lymph cells that produce antibodies or immunoglobulins
Mucus-Associated Lymphoid Tissue (MALT)	Lymphoid tissue located in mucous membranes, initiating immune responses
Immunoglobulin G (IgG)	Enhances phagocytosis, neutralizes toxins and viruses, protects fetus and newborn
Immunoglobulin A (IgA)	Provides localized protection on mucosal surfaces, fights antigens through agglutination
Immunoglobulin M (IgM)	Effective against microorganisms and agglutinating antigens, critical in complement activation
Immunoglobulin E (IgE)	Triggers the release of chemicals by eosinophils upon recognition of a pathogen
Immunoglobulin D (IgD)	Less is known about this antibody
Plasma Cells	Cells that secrete immunoglobulins, Y-shaped antibodies
T Lymphocytes	Lymph cells that directly attack pathogens
Memory B Cells	B cells with B cell receptors (BCRs) that trigger antibody release upon recognizing the antigen
Antibody Functions	Activating the complement system, neutralizing antigens, opsonization, killing by oxidation, agglutination, cellular cytotoxicity
Clonal Deletion	Process of eliminating B cells that are self-reactive or recognize self-antigens