Immunoglobulin Classes and Humoral vs Cell-mediated Immunity

Secreted in breast milk.
Provides mucosal immunity by acting at mucosal surfaces and contributing passive protection to the newborn.
Role highlighted by its presence in secretions (e.g., breast milk) rather than in large quantities in circulation.

A B cell receptor and not secreted.
Membrane-bound on the surface of naive B cells; functions as the antigen receptor.
Not typically released in substantial amounts into the bloodstream.

High in an allergic reaction.
Binds to Fc receptors on mast cells and basophils; cross-linking by an allergen leads to degranulation and release of histamine.
Also involved in defense against parasitic infections (typical secondary context in immunology).

Provides long term immunity.
Major antibody in the blood and extracellular fluid; participates in secondary immune responses.
Functions include neutralization of pathogens and toxins, opsonization, and activation of the complement system (classical pathway).
Can cross the placenta, contributing to fetal and neonatal immunity (contextual note).

First one secreted when exposed to an antigen.
Typically produced early in a primary immune response.
Often present as a pentamer on initial responses (high avidity, though individual affinity may be lower).
Strong activator of the complement system, contributing to initial pathogen clearance.

Immune response that mainly targets extracellular pathogens.
Antibodies coat pathogens to neutralize them or target them for phagocytosis (opsonization).
Key players: B-cells and memory cells; plasma cells; antibodies.
Functional implications:
- Neutralization of toxins and viruses outside cells.
- Opsonization enhances phagocytosis by macrophages and neutrophils.
- Complement activation via the classical pathway can lead to lysis of pathogens and enhanced opsonization.

Immune response that targets intracellular pathogens.
Targets infected cells for cellular death (apoptosis) to prevent replication and spread.
Key players:
- Antigen-presenting cells (APCs).
- T helper cells (CD4+ T cells).
- T-cells/memory cells.
- Cytotoxic T cells – $\text{CD8}^+$ .
- T helper cells – $\text{CD4}^+$ .
- Natural killer cells and macrophages.
- Digestion enzymes (e.g., within phagosomes of macrophages) as part of intracellular pathogen degradation.

Notes on connections and mechanisms

Humoral and cell-mediated arms interact: helper T cells (CD4+ T cells) activate B cells to form plasma cells and memory B cells, enhancing antibody production.
Cytotoxic T cells (CD8+): recognize antigen presented by MHC class I on infected cells and induce apoptosis to eliminate intracellular pathogens.
Antigen-presenting cells (APCs) bridge innate and adaptive immunity by processing pathogen-derived antigens and presenting them to T cells.
Memory cell formation in both branches enables faster and stronger responses upon subsequent exposures.

Important terminology recap

Antibodies: produced by B cells/plasma cells; facilitate neutralization, opsonization, and complement activation.
Antigen-presenting cells (APCs): include dendritic cells, macrophages, and B cells; present antigenic peptides to T cells.
CD8+ T cells: cytotoxic T lymphocytes that kill infected cells.
CD4+ T cells: helper T cells that coordinate immune responses and help B cells.
NK cells and macrophages: contribute to early defense and clearance of infected cells through cytotoxicity and phagocytosis, often interacting with digestion enzymes in intracellular pathways.