Adaptive Immunity Overview

Adaptive Immunity Overview

• Adaptive immunity is the body's ability to defend itself against specific invading agents.
• Antigens (Ags) are substances recognized as foreign, triggering an immune response.
• It differs from innate immunity through specificity and memory.

Maturation of T cells and B cells

• B cells mature in red bone marrow.
• T cells develop from pre-T cells that migrate from red bone marrow to the thymus.
• Helper T cells (CD4 T cells) and cytotoxic T cells (CD8 T cells) are crucial parts of the system.
• Immunocompetence is the ability to carry out an adaptive immune response.
• Antigen receptors are proteins embedded in plasma membranes, specifically recognizing antigens.

Types of Adaptive Immunity

• Cell-mediated immunity: Cytotoxic T cells directly attack invading agents, especially intracellular pathogens (like viruses, bacteria, and fungi), some cancer cells, and foreign tissue transplants.
• Antibody-mediated immunity: B cells transform into plasma cells, producing antibodies (Abs) or immunoglobulins. These target extracellular pathogens in body fluids. Helper T cells support both types of immunity.

Clonal Selection

• Lymphocytes proliferate and differentiate in response to a specific antigen, creating a clone of identical cells recognizing the same antigen.
• This process occurs in secondary lymphatic organs/tissues (e.g., lymph nodes, tonsils).
• Swollen lymph nodes/tonsils can indicate clonal selection.
• Effector cells (e.g., active helper T cells, cytotoxic T cells, plasma cells) are responsible for acting against the antigen. They are short-lived.
• Memory cells are long-lived; they participate in the secondary immune response, responding faster to a повторное invasion of the same antigen.

Antigens

• Antigens have two key characteristics: immunogenicity (ability to provoke an immune response) and reactivity (ability to react specifically with antibodies).
• The entire microbe may act as an antigen, but typically smaller parts like epitopes or antigenic determinants evoke the immune response.
• Different epitopes are recognized by the human immune system (a billion or more).
• Major Histocompatibility Complex (MHC) antigens (proteins) are located on the plasma membrane surfaces of most body cells. MHC or human leukocyte antigens (HLA) assist T cells in distinguishing foreign vs. self.

Pathways of Antigen Processing

• B cells recognize and bind to antigens in bodily fluids (lymph, interstitial fluid, blood plasma).
• T cells recognize processed and presented antigen fragments.
• Antigenic proteins are broken down into peptide fragments, associated with MHC molecules, and then presented on plasma membranes.
• The antigen processing pathway depends on whether the antigen is outside or inside body cells.

Antigen Presenting Cells (APCs)

• APCs (e.g., dendritic cells, macrophages, B cells) process and present the antigen.
• APCs are located in tissues or organs where antigens may penetrate, such as skin, respiratory and gastrointestinal tracts, urinary and reproductive tracts, lymph nodes.
• APCs migrate to lymph nodes after processing the antigen.

Exogenous and Endogenous Antigens

• Exogenous antigens are present in fluids outside body cells. Antigen-presenting cells (APCs) process and present these antigens to T cells using MHC-II molecules.
• Endogenous antigens are found inside body cells. Infected cells display these antigens using MHC-I molecules to T cells. After processing, APCs migrate to lymphatic tissues.

Cell-mediated Immunity (Activation of T cells)

• T-cell activation requires two signals: (1) TCR binding to specific antigen fragments presented on antigen-MHC complexes, and (2) costimulation from other molecules.
• Anergy is a state of inactivity that may occur if a T cell encounters an antigen without costimulation.

Activation and Clonal Selection of Helper T Cells

• Most CD4 T cells become helper T cells (CD4 T cells).
• They recognize exogenous antigens combined with MHC-II molecules on APCs.
• Helper T cells secrete cytokines like interleukin-2 (IL-2) that activate other immune cells and help them proliferate.
• Memory helper T cells quickly respond to further encounters.

Activation and Clonal Selection of Cytotoxic T Cells

• Most CD8 T cells become cytotoxic T cells (CD8 T cells).
• They recognize antigens combined with MHC-I molecules.
• They need costimulation.
• The activated cytotoxic T cells destroy infected body cells.
• Memory cytotoxic T cells keep a record for subsequent encounters.

Elimination of Invaders

• Cytotoxic T cells seek out and kill infected target cells.
• Granzymes and perforin/granulysin trigger apoptosis or cytolysis of infected cells.
• This is important in immunological surveillance, preventing tumors and other anomalies.