Immune System Overview

Immune System Overview

• The immune system is a collection of cells, tissues, organs, and molecules that protect against microorganisms, microbial toxins, and tumor cells.
• Lymphoid tissue and organs are divided into central (primary) and peripheral (secondary).

Central Lymphoid Organs

• Bone Marrow: Located within the central cavity of all bones, responsible for blood cell production (hematopoiesis) and development of myeloid and lymphoid cells.

• Thymus Gland: A small gland behind the sternum, facilitating the development and proliferation of T lymphocytes (T cells), differentiating them into cytotoxic or helper T cells.

Peripheral Lymphoid Organs

• Lymph Nodes: Filter lymph, crucial for immune responses.

• Spleen: Filters blood, involved in immune responses to blood-borne pathogens.

• Mucosa-associated lymphoid tissues (MALT): Including tonsils, adenoids, Peyer's patches, and appendix, these tissues are crucial for defending mucosal surfaces.

Immune System Molecules

• Antibodies (immunoglobulins): Proteins produced by B lymphocytes (plasma cells) that specifically target antigens.

• Lymphokines: Cytokines secreted by T lymphocytes, influencing immune responses through signaling between immune cells.

Immune System Cells

Lymphocytes

• B lymphocytes: Produced and mature in the bone marrow, expressing unique antigen-binding receptors (B cell receptors). Upon antigen exposure and cytokine stimulation, most B cells become plasma cells, which produce antibodies. Some become memory B cells for future encounters.

• T lymphocytes: Produced in bone marrow but mature in the thymus, not producing antibodies. They recognize antigens through cell surface MHC receptors.

• Cytotoxic T cells (CD8+): Recognize antigens presented on MHC I molecules, primarily on virus-infected cells and kill them.

• Helper T cells (CD4+): Crucial for both humoral and cellular immune responses. Recognize antigens presented on MHC II molecules and secrete cytokines to activate B cells and other immune cells.

• Two major types:

• Th1 cells: Produce IFN-γ to activate cytotoxic T cells.

• Th2 cells: Produce IL-4 and IL-5 to activate B cells.

• Regulatory T cells (Treg): Suppress immune responses to prevent overactivity.

• Memory T cells: Recognize antigens from previous encounters, initiating rapid responses upon re-exposure.

Other Cells

• Natural Killer (NK) cells: Large granular lymphocytes capable of killing virus-infected cells or tumor cells without prior sensitization. NK cell activity depends on a balance between activating and inhibitory signals.

• Granulocytes (e.g., neutrophils): First responders to infections, possessing bactericidal activity. Neutrophils accumulate at sites of inflammation and form pus. Eosinophils are involved in parasitic infections, Basophils are involved in allergic reactions.

• Monocytes/Macrophages: Migrate into tissues and differentiate into macrophages, performing phagocytosis and presenting antigens to T cells, essential for initiating specific immune responses.

• Dendritic Cells: Phagocytose antigens and present them to T lymphocytes, facilitating the initiation of adaptive immunity, located in tissues and lymphatic areas.

• Antigen-presenting cells: Macrophages, Dendritic cells, and B cells are crucial for initiating adaptive immunity by capturing, processing, and presenting antigens to T cells.