Immunology: MALT and Major Organs

Regional Immunity (MALT)

• MALT is a key component of the immune system, vital for defending mucosal surfaces.
• Mucosal surfaces are large, exposed areas susceptible to pathogens.

Gastrointestinal Tract

• Special challenges include tolerance of food antigens and commensal microbiota, responsiveness to potential pathogens, and an extensive surface area.
• Specialized cells help regulate immune tolerance, and handle a diverse array of microorganisms.

Respiratory System

• The respiratory system faces a constant barrage of inhaled pathogens and innocuous microbes.
• Ciliary movement and mucus production clear particles and microbes.

Cutaneous Immune System

• Large surface area exposed to microbes and pathogens.
• Keratinizing stratified squamous epithelial barrier provides a protective layer.

Major Lymphoid Organs and Tissues

• Primary lymphoid organs include the thymus and bone marrow.
• Secondary lymphoid organs include lymph nodes, spleen, tonsils, adenoids, Peyer's patches, and bronchus-associated lymphoid tissue (BALT).

Mucosal Immune System

• It's a crucial first line of defense against pathogens that enter the body through mucosal surfaces.
• This system consists of secretory IgA, dendritic cells, T cells, and B cells, effectively defending against microbes, viruses, and allergens.
• The total surface area of the mucosal immune system is about 400 m², significantly larger than the skin's.

MALT Regions

• GALT: Gut Associated Lymphoid Tissue
• BALT: Bronchus Associated Lymphoid Tissue
• NALT: Nasal Associated Lymphoid Tissue
• RALT: Reproductive Associated Lymphoid Tissue

GALT (Peyer's Patch)

• GALT (Peyer's Patch) is a critical component in the gut-associated lymphoid tissue, initiating immune responses.
• T cells, B cells, M cells, and dendritic cells are important participants in initiating and coordinating immune responses.

PP Structure

• Peyer's Patch (PP) is principally composed of three functional regions:
• Dome Region
• B Lymphoid Region (Follicular Germinal Centers)
• T Lymphoid Region (Parafollicular Zone)

GALT Structure and Function (Immune Compartments)

• Induction Sites: Peyer's patches, appendix, Mesenteric lymph nodes, other lymph nodes deliver antigen to start immune response.
• Effector Sites: Lamina propria (beneath gut epithelium).
• These sites work in tandem to initiate and carry out mucosal immune responses.

M Cells in the Small Intestine

• M cells are specialized epithelial cells found in the intestinal lining.
• They sample and deliver antigens from the intestinal lumen to the underlying immune system.

Cells in the GALT (Epithelial, γδ T, αβ T)

• Epithelial cells and innate lymphoid cells contribute to mucosal immunity by creating an interface between the gut and external environment.
• γδ T cells play specialized roles in early immune responses and antigen presentation.
• αβ T cells and B cells coordinate the specific adaptive immune response for antigen surveillance.

Innate Immunity in Gastrointestinal Tract

• Intestinal epithelial cells are essential for the innate immune response in the gastrointestinal tract.
• These cells are involved in immune responses, tolerance to commensal organisms, and antigen sampling for the adaptive immune system.
• Mucosal layers prevent pathogens from coming into contact with epithelial cells.

Role of Th1 and Th2 cells

• Th1 and Th2 cells are crucial for producing mucosal IgA, a key antibody for defense in mucosal surfaces.
• These cells play a crucial role in initiating the IgA response.

IgA1 and IgA2 Antibody Producing Cells

• Different mucosal sites have varying proportions of IgA1 and IgA2 producing cells.
• This demonstrates the localized and specific nature of mucosal immune responses.

Lymphocyte Circulation

• Lymphocytes continuously circulate between mucosal tissues, lymphatic vessels, and the bloodstream to maintain immune surveillance.
• This circulation ensures that immune cells are readily available for antigen detection and response.