General Immunology - The Immune System and MHC

A collection of vocabulary flashcards covering the fundamental concepts of immunology, including MHC structures, lymphoid organs, and various immune cell types based on the lecture notes.

Immune System

A set of organs and elements scattered throughout the body involved in the defence against the ‘non-self.’

Self

The set of molecules resulting from the expression of an individual's genome, defined primarily by cellular markers on cell membranes.

Non-self

The set of molecules other than the self which trigger immune reactions when present in the body, such as bacteria, viruses, or modified self-molecules like cancer.

Major Histocompatibility Complex (MHC)

A group of polymorphic membrane glycoproteins, also known as HLA in humans and H2 in mice, that varying widely in composition and cause rejection reactions during transplantation.

MHC Class I Molecules

Molecules present on all nucleated cells, formed of a heavy $\alpha$ chain with 3 domains and a light $\beta_2$ microglobulin chain.

MHC Class II Molecules

Molecules present on antigen-presenting cells (APCs) such as B lymphocytes, macrophages, and dendritic cells, composed of an $\alpha$ chain and a $\beta$ chain.

Antigen Processing

The breakdown of proteins into smaller peptide fragments by immune cells to be presented on the cell surface.

Endogenous Pathway

The pathway for intracellular antigens (viral or tumor) where proteins are degraded by the proteasome and transported by TAP to the ER to bind with MHC class I.

Exogenous Pathway

The pathway for extracellular antigens (bacterial toxins) taken up by phagocytosis, broken down in lysosomes, and loaded onto MHC class II in MIICs.

Primary Lymphoid Organs

Organs that produce, proliferate, and mature lymphocytes, specifically the bone marrow and the thymus.

Secondary Lymphoid Organs

Sites where antigens and lymphocytes interact, including lymph nodes, the spleen, and MALTs.

Bone Marrow

The tissue in the central part of short and flat bones where hematopoietic stem cells (HSCs) produce blood cell lines.

Bursa of Fabricius

A lymphoid organ found exclusively in birds, located near the cloaca, crucial for the formation and maturation of B lymphocytes.

Thymus

A lymphoepithelial organ in the mediastinum where T lymphocyte precursors mature and multiply; it undergoes progressive involution after puberty.

Positive Selection

The process in the thymic cortex where T cells that recognize the MHC-peptide complex are selected to survive.

Negative Selection

The process in the thymic medulla where T cells reacting strongly to self-peptides are eliminated to maintain self-tolerance.

Spleen

An abdominal organ that acts as a blood filter, harboring red pulp for RBC regulation and white pulp for antigen-lymphocyte encounters.

Lymph Nodes

Contact zones distributed in a network throughout the body that filter lymph to stop cancer cells or germs.

MALT

Mucosa Associated Lymphoid Tissue, which defends entry sites in the digestive, respiratory, and urogenital systems, covering an area of approximately $400~m^2$.

Clusters of Differentiation (CD)

Surface proteins expressed on immune cells that serve as diagnostic markers for immunophenotypic identification.

Monocytes

The largest leukocytes ($20\text{ to }25~\mu m$ in diameter) that differentiate into macrophages or dendritic cells once they reach tissues.

Macrophages

The phagocytic cell par excellence, ubiquitous in tissues (e.g., Kupffer cells in the liver), which acts as an antigen-presenting cell.

Dendritic Cells

Cells with cytoplasmic expansions that activate lymphocytes in secondary lymphoid organs and maintain self-tolerance in the thymus.

Polynuclear Neutrophils

The most numerous granulocytes in the blood, characterized by a multilobed nucleus and the ability to produce NETs (Neutrophil Extracellular Traps).

NETosis

The process where neutrophils release decondensed chromatin (DNA) and antimicrobial enzymes to trap and neutralize pathogens externally.

Polynuclear Basophils

The least common granulocytes (less than $1\%$) which play a critical role in allergic responses by releasing histamine via surface IgE receptors.

Polynuclear Acidophils (Eosinophils)

Cells with bilobed nuclei and reddish-orange granules specialized for defense against parasites and modulating long-term allergic reactions.

Mast Cells

Tissue-resident cells containing histamine and heparin that express Fc receptors for IgE and can trigger anaphylactic shock during degranulation.

Natural Killer (NK) Cells

Large granular lymphocytes characterized by $CD56$ that kill infected or tumor cells via direct killing (perforin/granzymes) or ADCC.

B Lymphocytes (LB)

Cells of the adaptive immune response that mature in the bone marrow and differentiate into antibody-secreting plasma cells or memory B cells.

T Lymphocytes (LT)

Cells responsible for cell-mediated immunity characterized by $CD3$ and the TCR; they differentiate into $CD4^+$ helper cells or $CD8^+$ cytotoxic cells.

Complement System

A series of plasma proteins that function in a cascade to destroy pathogens via the membrane attack complex and promote opsonization.

Cytokines

Small proteins such as interleukins, interferons, and TNF that act as molecular messengers to coordinate immune functions.

Chemokines

A subset of cytokines that create a chemical gradient to guide the migration of immune cells to sites of infection or injury.

Antibodies

Molecules produced by plasma cells that specifically recognize foreign antigens and trigger mechanisms for their destruction.