Adaptive Immunity – Humoral (antibody-mediated) mechanisms

Flashcards covering concepts of adaptive immunity, humoral immunity, antigens, lymphocytes, antibody structure, and functions based on Chapter 21 lecture notes.

What are the characteristics of the adaptive immune system?

It protects against infectious agents and abnormal cells, amplifies the inflammatory response, activates complement, is specific, systemic, and has memory, requiring initial priming.

What are the two overlapping arms of the adaptive immune system?

Humoral (antibody-mediated) immunity and Cellular (cell-mediated) immunity.

How does Humoral Immunity work?

Antibodies produced by B lymphocytes circulate freely, bind temporarily to target cells, inactivate them, and mark them for destruction by phagocytes or complement. It targets extracellular pathogens.

How does Cell-mediated Immunity work?

T lymphocytes act directly by killing infected cells or indirectly by releasing chemicals that enhance the inflammatory response or activate other lymphocytes or macrophages. It targets cellular pathogens like virus-infected cells, cancer cells, and foreign grafts.

What are non-self antigens?

Substances that can mobilize adaptive defenses, provoke an immune response, and are typically large, complex molecules not normally found in the body.

What is immunogenicity?

The ability of a substance to stimulate the proliferation of specific lymphocytes.

What is reactivity?

The ability of a substance to react with activated lymphocytes and antibodies released by immunogenic reactions.

What are antigenic determinants?

Specific parts of an entire antigen that are immunogenic and where antibodies and lymphocyte receptors bind.

What are MHC proteins (Major Histocompatibility Complex)?

Self-antigens on the surface of cells, unique to an individual, that present self- or foreign antigens.

Why are MHC proteins important for T lymphocytes?

T lymphocytes can only recognize antigens that are presented on MHC proteins.

What are the three types of cells comprising the adaptive immune system?

B lymphocytes (B cells), T lymphocytes (T cells), and Antigen-presenting cells (APCs).

What are the five general steps of lymphocyte development, maturation, and activation?

Origin, maturation, seeding secondary lymphoid organs and circulation, antigen encounter and activation, and proliferation and differentiation.

Where do B and T lymphocyte precursors originate?

Red bone marrow.

Where do T cells mature?

Thymus.

Where do B cells mature?

Bone marrow.

What is immunocompetence?

The ability of a lymphocyte to recognize and bind to one specific non-self or foreign antigen.

What is self-tolerance in lymphocytes?

The state where lymphocytes are unresponsive to 'self' antigens and should not bind them.

What is positive selection in T cell maturation?

The process where T cells capable of recognizing self-MHC proteins (MHC restriction) are selected to survive, while those that fail are destroyed.

What is negative selection in T cell maturation?

The process that prompts apoptosis of T cells that bind tightly to self-antigens displayed by self-MHC, ensuring self-tolerance.

What does 'naïve' mean in the context of B and T cells?

Immunocompetent B and T cells that have not yet been exposed to a foreign antigen.

What is clonal selection?

The process where a naïve lymphocyte's first encounter with an antigen selects it for further development, leading to differentiation if correct signals are present.

What is the outcome of proliferation of activated lymphocytes?

They differentiate into effector cells that fight infections and memory cells that can respond more quickly to future exposures to the same antigen.

What type of immune response do B lymphocytes primarily mediate?

Humoral immunity.

What are the primary targets of B lymphocytes?

Extracellular pathogens (e.g., bacteria, fungi, parasites, some viruses in extracellular fluid).

What are the primary targets of T lymphocytes?

Intracellular pathogens (e.g., virus-infected cells) and cancer cells.

What are the effector cells for B lymphocytes?

Plasma cells.

What are the effector cells for T lymphocytes?

Cytotoxic T (TC) cells, Helper T (TH) cells, and Regulatory T (TReg) cells.

What is the main function of Antigen-presenting Cells (APCs)?

To engulf antigens and present fragments of them to T cells for recognition.

What are the major types of APCs?

Dendritic cells, Macrophages, and B cells.

Which type of APC is considered the most effective antigen presenter and a key link between innate and adaptive immunity?

Dendritic cells.

How is a B cell activated?

When antigens bind to its surface receptors and cross-link them, followed by receptor-mediated endocytosis of the antigen-receptor complexes.

What do most activated B cells differentiate into?

Plasma cells.

What is the function of plasma cells?

They secrete specific antibodies at a rapid rate, which circulate in the blood or lymph to bind free antigens and mark them for destruction.

What is the function of memory B cells?

They provide immunological memory and mount an immediate response to future exposures to the same antigen.

What characterizes a primary immune response?

Cell proliferation and differentiation upon first exposure to an antigen, with a lag period of three to six days and antibody levels peaking around 10 days before declining.

What characterizes a secondary immune response?

A faster, more prolonged, and more effective response upon re-exposure to the same antigen, with sensitized memory cells responding within hours and antibody levels peaking in two to three days at much higher levels.

What is active humoral immunity?

Immunity gained when B cells encounter antigens and produce specific antibodies against them.

What are the two types of active humoral immunity?

Naturally acquired (response to bacterial or viral infection) and artificially acquired (response to a vaccine).

How do vaccines work?

They provide dead or attenuated pathogens that stimulate a primary immune response, sparing symptoms, and providing immunogenic and reactive antigenic determinants.

What is passive humoral immunity?

Immunity gained due to readymade antibodies introduced into the body, where B cells are not challenged, and no immunological memory is formed.

Does passive immunity provide immunological memory?

No, it does not, and protection ends when antibodies degrade.

What are the two types of passive humoral immunity?

Naturally acquired (antibodies delivered to fetus via placenta or to infant through breastmilk) and artificially acquired (injection of serum, such as gamma globulin).

What are antibodies also known as?

Immunoglobulins (Ig).

What is the basic structure of an antibody monomer?

A T- or Y-shaped molecule of four looping polypeptide chains (two identical heavy chains and two identical light chains) linked by disulfide bonds, with variable regions forming antigen-binding sites and constant regions determining the antibody class.

Which part of an antibody determines its class (IgM, IgA, IgD, IgG, or IgE)?

The constant (C) regions of the stem.

What is the first immunoglobulin class secreted during the primary response, and what does its presence usually indicate?

IgM; its presence in plasma usually indicates a current infection by the pathogen eliciting its formation.

Which immunoglobulin is found in body secretions like saliva, sweat, and milk and helps stop pathogens from attaching to epithelial cell surfaces?

IgA (as a dimer, secretory IgA).

Which immunoglobulin functions as a B cell antigen receptor?

IgD (and monomeric IgM).

Which immunoglobulin is the most abundant antibody in plasma and passes from mother to fetus?

IgG.

Which immunoglobulin binds to mast cells or basophils and triggers the release of histamine, mediating inflammation and allergic reactions?

IgE.

Do antibodies destroy antigens directly? If not, what do they do?

No, antibodies inactivate and tag antigens; they form antigen-antibody (immune) complexes.

What are the four defensive mechanisms used by antibodies?

Neutralization, Agglutination, Precipitation, and Complement fixation.

Describe neutralization as an antibody mechanism.

Antibodies block specific sites on viruses or bacterial exotoxins, preventing them from binding to tissue cells, and the resulting complexes undergo phagocytosis.

Describe agglutination as an antibody mechanism.

Antibodies bind to the same determinant on more than one cell-bound antigen, cross-linking them to cause clumping (e.g., of mismatched blood cells).

Describe precipitation as an antibody mechanism.

Soluble molecules are cross-linked by antibodies, forming insoluble complexes that precipitate out and are subject to phagocytosis.

Describe complement fixation as an antibody mechanism.

Several antibodies bind close together on a cellular antigen, triggering complement fixation into the cell's surface, leading to cell lysis (possibly via MAC), amplification of inflammatory response, and promotion of phagocytosis via opsonization.