Lymphoid Tissue

Lecture six

Lymphoid tissue
By Dr.Alaa Al-sahlany

Lymphoid system consists of tissues and organs mainly made of lymphocytes, which protect the body against invasion of microorganisms

Cells of the lymphoid System

Lymphocytes:
1. B lymphocytes
2. T lymphocytes
2. Natural killer (NK) cells
Supporting cells:
1. Macrophages
2. Antigen-presenting cells (APC) such as Langerhans cell in skin.
3. Neutrophils.
4. Basophils and eosinophils

Classification of Lymphoid Tissue

1.Diffuse lymphoid tissue (Mucosa associated lymphoid tissue MALT)
Lymphocytes are deep to epithelium in lamina propria of digestive, respiratory tract




2.Dense lymphoid tissue
lymphocytes arranged in the form of nodules

Dense lymphoid tissue

(A) Non-encapsulated: e.g. Aggregated nodules (Peyers patches)


(B)Encapsulated discrete lymphoid organs: includes
Thymus
Lymph node
Spleen

Thymus

Located behind the sternum and in front of the heart
Made up of two lobes, each consisting of a central medulla and outer cortex surrounded by capsule

Composition

Connective tissue capsule: covers the thymus and sends septae into the interior dividing the lobe into lobules.
Parenchyma: Each lobule has a darkly stained cortex at the periphery and a lightly stained medulla in the centre that has Hassalls corpuscles

FUNCTIONS

It is a central lymphoid organ and is essential till puberty for T cells differentiation and maturation. After puberty it undergoes involution.
Secrete hormones like thymopoietin, etc., which are involved in stimulation and differentiation of T lymphocytes(no B lymphocytes).
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LYMPH NODE




Bean-shaped structures situated along the course of the lymphatic vessels
Usually found in groups such as in the axilla, inguinal region
Function
Localizing and preventing the spread of infection (defense).

Composition

A. Connective tissue framework
The organ is surrounded by a connective tissue capsule which sends trabeculae into the interior.

B. Parenchyma

1.Cortex : is the peripheral part of the lymph node. It contains:
(a) Subcapsular sinus
(b) Lymphatic follicleswith or without germinal centers and these follicles are formed mainly of B lymphocytes.

2.Paracortex(deep cortex)

It consists mainly of T lymphocytes . It is part of cortex lie between lymphatic follicle and medulla and it does not have follicles

3.Medulla: consist of

(a)The medullary cords are composed of closely packed lymphocytes
(b)The medullary sinuses lie between the medullary cords.
Medullary sinus are lightly stained compared to the darkly stained medullary cords

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SPLEEN





Is the largest lymphoid organ
located in the upper left quadrant of the abdominal cavity

Composition

A)Connective tissue framework
Covered by capsule which send trabeculae that carry trabecular vessels

B) Parenchyma

(1) White pulp: mainly consist of lymphocytes
Composed of two parts:
a)Periarterial lymphatic sheath: made mainly of T lymphocytes that aggregate around the central artery(a branch of splenic artery)
b)Lymphatic follicles :formed mainly of B lymphocytes

(2)Red pulp : heavily infiltrated with all the cells of the circulating blood including large number of RBC, giving a dark red color to this tissue
Consists of two parts:
a)Splenic sinuses (which are sinusoidal capillaries)
b)Splenic cords ( that has RBC and all types of WBC including macrophage )


Function
Filtration of blood: filters the blood from antigens, microorganisms, aged platelets and aged and abnormal RBCs
Production of lymphocytes (defense of the body)
Acts as haemopoietic organ (in fetal life).

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