Lymph Node

 Lymph nodes are small bean-shaped organs; they occur in clusters or in chains, distributed along the length of lymphatic vessels. They act as physiological barriers; filter the microbial antigens carried to lymph node by activating the T and B cells.

Structure

Lymph node is divided into three parts—(1) cortex (2) medulla (both are B cell areas) and (3) paracortex (T cell area). It bears the lymphatic and blood vessels. Cortex is surrounded by a capsule and intervened by trabeculae (Fig. 1).

Fig1. Lymph node.

* Cortex: It contains lymphoid follicles that are composed of mainly B cells and few special type of dendritic cells (called follicular dendritic cells). Lymphoid follicles are mainly of two types

- Primary lymphoid follicles: They are found before the antigenic stimulus. They are smaller in size and mainly contain the resting B cells

- Secondary lymphoid follicles: Following contact with an antigen, the resting B cells start dividing and become activated. The activated B cells differentiate rapidly into plasma cells (which produce antibodies) and memory B cells (which become activated on subsequent antigenic exposure). Follicles become larger in size and are called secondary lymphoid follicles. They have two areas:

1- The central area called germinal center; contains dividing B cells of various stages. It has two zones—light and dark zones. It is the site where activation of B cells takes place

2- The peripheral zone called mantle area; contains activated B cells.

- Paracortical area: It is present in between cortex and medulla. It is the T cell area of lymph node; rich in naive T cells. In addition, it also contains macrophages and interdigitating dendritic cells, which trap the antigens and present to T cells

- Medulla: It is the innermost area of lymph node, rich in B-lymphocytes; mainly plasma cells.

Spleen

Spleen is the largest secondary lymphoid organ. It acts as physiological barrier similar to lymph node in clearing the microbial antigens through the stimulation of T and B cells.

Structure

 Spleen is situated below the diaphragm on left side of the abdomen. Adult spleen measures about 5-inch in length and weighs 150 g. It is divided into two compartments—(1) central white pulp and (2) outer red pulp, surrounded by capsule and intervened by trabeculae (Fig.2).

Fig2. Spleen (cross-section showing red pulp and white pulp area).

* White pulp: It is the central densely populated area, which contains T cells and B cells. It has two parts:

- Periarteriolar lymphoid sheath (PALS): It is T cell area, rich in T cells; surrounds the branches of the splenic artery

- Marginal zone: It is located peripheral to the PALS and is populated by B cell lymphoid follicles (primary and secondary) and macrophages.

* Red pulp: It is the area that surrounds the sinusoids. It is filled with red blood cells (RBCs). The older and defective RBCs are destroyed here.

Defect in Spleen

 As spleen is the site of destruction of most of the microbes, functional or structural abnormalities of spleen or splenectomy, especially in children, often leads to an increased incidence of bacterial sepsis caused primarily by capsulated bacteria such as Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae.

Mucosa-associated Lymphoid Tissue (MALT)

 The mucous membranes lining the intestine, respiratory, and urogenital tract (total surface area of about 400 m2) are the major sites of entry for most pathogens. Hence, defense mechanisms are needed in the mucosal sites to prevent the microbial entry.

* Group of lymphoid tissues lining these mucosal sites are collectively known as MALT

* Structurally, MALT may be arranged in two types:

- Loose clusters of lymphoid cells (usually found in the lamina propria of intestinal villi)

- Lymphoid tissues arranged as organized structures such as tonsils, appendix and Peyer’s patches).

MALT in Intestinal Mucosa

Lymphoid tissues lining the intestinal mucosa are the best studied MALT. They are present in different layers of wall.

* MALT in Intestinal Mucosa Lymphoid tissues lining the intestinal mucosa are the best studied MALT. They are present in different layers of wall.

* Lamina propria contains loose clusters of lymphocytes (B cells, plasma cells, T helper cells) and macrophages

* Epithelial layer contains few specialized lymphocytes called intraepithelial lymphocytes (IELs) and modified epithelial cells (called M cells)

- Intraepithelial lymphocytes (IELs) are the γδ T cells. The actual function of such T cells is not known, they may encounter the lipid antigens that enter through the intestinal mucosa

- M cells: Described in the box below

- Secretory IgA: These are the dimeric IgA antibodies that are present in the submucosa as well as in the lining epithelium. They prevent the microbial entry at the mucosal sites (local immunity).

Cutaneous-associated Lymphoid Tissue

 Similar to MALT, skin also contains a few loose lymphocytes and specialized antigen presenting cells in epidermis called Langerhans cells.

اخر الاخبار

اخبار العتبة العباسية المقدسة

تزامنًا مع حلول شهر ربيع الأول.. العتبة العباسية المقدسة ترفع مظاهر الحزن والسواد في المرقد الطاهر

بعد انتهاء شهري المحرم وصفر.. قسم الصيانة يباشر إدامة الثريات واستبدال مصابيح المرقد الطاهر

قسم الشؤون الخدمية يشرع بإدامة آليّات النظافة عقب انتهاء زيارة الأربعين

موكب عزاء الشعراء والرواديد ينظم مسيرة عزائية في ختام شهري الأحزان المحرم الحرام وصفر

المزيد

الآخبار الصحية

العنب.. فاكهة صغيرة بمفعول "خارق" للقلب والدماغ

دراسة تحدد "الحلقة المفقودة".. تسبب ارتفاع ضغط الدم

هذه الأطعمة تحمي دماغك من الشيخوخة وتزيد حدة الذكاء

خبراء يحذرون من مخاطر "الدهون المخفية" على القلب

المزيد

الآخبار التكنلوجية

الأرصاد الجوية تلجأ إلى الذكاء الاصطناعي.. لأي غرض؟

في منزلك.. "تعديل بسيط" يخفض ضغط الدم بشكل طبيعي

"ميزة جديدة" من إنستغرام تنافس تيك توك.. طال انتظارها

غوغل في مأزق.. بيانات 2.5 مليار مستخدم بقبضة القراصنة

المزيد

مواضيع ذات صلة

Central Lymphoid Organs

Effector Functions of Complement

Complement Pathways

Mechanisms of innate immunity

Immunologic Manifestations of Viral, Rickettsial, and Mycoplasmal Diseases

Immunologic Manifestations of Fungal Diseases

The Structure of Antibodies

Myelomas and Monoclonal Antibodies

Telling the Difference Between Foreign and Self

The Basic Adaptive Immune Response

The Immunoglobulin Domain and Variations

Deficiencies of the Immune Response