Immunity -types -components of immune system (mps,lymphoid component)

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Here is a comprehensive overview of Immunity — Types and Components of the Immune System, drawn from authoritative immunology and histology textbooks.

IMMUNITY

1. Types of Immunity

Defense against microbes is mediated by two sequential, coordinated arms:

Innate vs. Adaptive immunity — Cellular and Molecular Immunology

A. Innate Immunity (Natural / Native Immunity)

Feature	Detail
Speed	Responds within hours of infection
Specificity	Recognizes molecular patterns shared by groups of related microbes (PAMPs)
Diversity	Low — encoded by inherited (germline) genes
Memory	Limited or absent
Self-discrimination	Yes

Principal components:

Physical & chemical barriers — skin, mucosal epithelia, antimicrobial peptides
Cellular elements — phagocytes (macrophages, neutrophils), dendritic cells (DCs), mast cells, natural killer (NK) cells, and other innate lymphoid cells (ILCs)
Soluble proteins — complement system, lectins, acute-phase proteins, interferons

B. Adaptive Immunity (Specific / Acquired Immunity)

Feature	Detail
Speed	Days (develops after innate response primes it)
Specificity	For individual microbial and non-microbial antigens (epitopes)
Diversity	Very high — somatic recombination of gene segments in lymphocytes generates 10⁷–10⁹ distinct specificities
Memory	Yes — stronger, faster responses on re-exposure
Self-discrimination	Yes

Two forms of adaptive immunity:

	Humoral Immunity	Cell-Mediated Immunity
Mediator cell	B lymphocytes	T lymphocytes
Effector molecule	Antibodies (immunoglobulins)	Cytokines, cytotoxic killing
Target	Extracellular microbes and toxins	Intracellular pathogens, virus-infected cells

Active vs. Passive immunity:

Active — host responds to an antigen (natural infection or vaccination)
Passive — transfer of preformed antibodies (e.g., maternal IgG, antitoxin injection)

2. Components of the Immune System

All immune cells arise from pluripotent hematopoietic stem cells via two main lineages:

Myeloid lineage → monocytes, macrophages, neutrophils, dendritic cells, mast cells, basophils, eosinophils
Lymphoid lineage → T cells, B cells, NK cells

A. Mononuclear Phagocyte System (MPS)

The MPS is a network of phagocytic cells of myeloid origin primarily involved in phagocytosis, antigen processing, and antigen presentation. It comprises monocytes, macrophages, and dendritic cells.

Origins (Two Distinct Waves)

Early (primitive) wave — Erythro-myeloid progenitors from the yolk sac (~3–4 weeks gestation). These become resident tissue macrophages that are self-renewing and independent of blood monocytes.
Definitive wave — Bone marrow hematopoietic stem cells → monocytes → monocyte-derived macrophages (appear ~17th week gestation).

Cells of the MPS

Cell	Location
Monocytes (+ precursors: monoblasts, promonocytes)	Blood and bone marrow
Connective tissue macrophages (histiocytes)	Loose connective tissue, spleen, lymph nodes, bone marrow, adipose tissue, thymus
Dendritic cells	Lymph nodes, spleen
Kupffer cells (stellate sinusoidal macrophages)	Liver
Alveolar macrophages	Lungs
Microglia	Central nervous system
Langerhans cells	Epidermis of skin, oral mucosa
Mesangial cells	Kidney
Placental macrophages (Hofbauer cells)	Placenta
Osteoclasts	Bone
Intestinal macrophages	GI submucosa and lamina propria
Multinucleated giant cells (foreign-body, Langhans)	Pathologic granulomas

Key Functions of MPS Cells

Phagocytosis of pathogens and debris
Antigen processing and presentation via MHC class II (to CD4⁺ T helper cells)
Cytokine and chemokine secretion to recruit and regulate other immune cells
Express pattern recognition receptors (PRRs), complement receptors, and Fc receptors

Monocytes (10–18 μm, kidney-bean nucleus) make up 3–8% of peripheral blood leukocytes. They follow neutrophils into inflamed tissue and differentiate into macrophages or DCs.

B. Lymphoid Component

1. T Lymphocytes

Develop from bone marrow stem cells; mature in the thymus
Bear the T cell receptor (TCR) associated with the CD3 complex
CD4⁺ T helper cells — recognize antigen on MHC class II; help B cells and activate macrophages
CD8⁺ Cytotoxic T cells — recognize antigen on MHC class I; kill infected/tumor cells
Regulatory T cells (Tregs) — suppress immune responses; maintain self-tolerance

2. B Lymphocytes

Develop and mature in the bone marrow
Express surface immunoglobulin (B cell receptor, BCR)
On activation, differentiate into plasma cells (secrete antibodies) and memory B cells
Responsible for humoral immunity

3. Natural Killer (NK) Cells

Neither T nor B cells — innate lymphoid cells
Kill virus-infected cells and tumor cells without prior sensitization
Activated by absence of self-MHC class I (missing-self recognition)
Express CD16 (FcγRIII) → mediate antibody-dependent cellular cytotoxicity (ADCC)

4. Key Surface Markers Summary

Marker	Cell	Function
CD3	T cells	TCR signaling complex
CD4	T helper cells, monocytes	MHC class II receptor
CD8	Cytotoxic T cells	MHC class I receptor
CD19/CD21	B cells	B cell receptor complex
CD16/CD56	NK cells	Cytotoxicity, ADCC
CD14	Monocytes/macrophages	LPS receptor
CD80/CD86 (B7)	DCs, macrophages, B cells	T cell co-stimulation

C. Lymphoid Organs

Type	Organs	Function
Primary (central)	Thymus, Bone marrow	Site of lymphocyte development and maturation
Secondary (peripheral)	Lymph nodes, Spleen, MALT (tonsils, Peyer's patches)	Site of adaptive immune responses; antigen presentation to lymphocytes

Summary Diagram

B cells, helper T cells, cytotoxic T cells, and regulatory T cells — Cellular and Molecular Immunology

Sources:

Cellular and Molecular Immunology, Abbas & Lichtman (9780323757485)
Histology: A Text and Atlas with Correlated Cell and Molecular Biology (9781975181512)
Medical Microbiology, 9e (9780323673228)
Murray & Nadel's Textbook of Respiratory Medicine (9780323655873)

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