Question

1. Describe the different histological cells that play a role in immunity? What are Pyer’s patches?

2. What type of cells play a role in immunity in spleen and thymus? What are Hassel corpuscles?

Answer 1

1. There are two different types of lymphocytes,
T lymphocytes that are associated with cell-mediated immunity and B lymphocytes that are associated with humoral immunity. both cells arise from stem cells in bone marrow. Immature T lymphocytes migrate from the bone marrow into the thymus and become thymus-dependent or mature T lymphocytes. These thymus-dependent or mature T lymphocytes next move through the passage. And, they find homes in lymph nodes, mucosa-associated lymphoid tissue or the spleen. There are various types of T lymphocytes

• T helper cells
• cytotoxic T cells
• suppressor T cells

B lymphocytes originate and mature in the bone marrow then pass by circulating into different sites throughout the body. when they interact with the foreign antigen with the help of T helper cells, B lymphocytes become mature antibody secreting cells called plasma cells. plasma cells are found in connective tissue underneath epithelia, in the white pulp of the spleen and in the lymph nodes. here, they come in contact with foreign substances. These cells express one of the first lines of defence toward attacking microorganisms, viruses and parasites.
Peyer’s patches are small expanses of lymphatic tissue located throughout the ileum section of the small intestine. they are also known as aggregated lymphoid nodules, they monitor intestinal bacteria populations and prevent the growth of pathogenic bacteria in the intestines thereby playing important role in immune system.
2. Immature T lymphocytes migrate from the bone marrow into the thymus where they mature into immunocompetent T cells. These T cells then leave the thymus and circulate and ultimately find their way to lymph nodes, mucosa-associated lymphoid tissue or the spleen.
in Thymus the cells Functions by:

• producing immunocompetent T lymphocytes
• production and circulation of mature T cells in peripheral tissues
• the cells increase immunological self-tolerance
• T cell maturation, proliferation and function through secretion of hormones are regulated

Spleen:
The red pulp of the spleen consists of macrophages in parenchyma which are enclosed in capillaries and other macrophages and blood cells which have not entered the venous sinuses yet.
White Pulp:
The white pulp of the spleen is distinguished by a parenchyma that consists of two types of lymphocytes B cells and T cells. B cells are found in the lymphoid follicles spread everywhere in the organ. it helps in the initiation of immune responses by B cells to foreign antigens in the blood. T cells are located neighbouring the central arteries and compose a kind of sheath. This site is known as the periarteriolar lymphoid sheath.
Hassall's corpuscles or thymic corpuscles are found in the medulla of the thymus. Hassall's corpuscles are formed from eosinophilic type VI epithelial reticular cells. The cells can be spherical or ovoid and they are arranged concentrically. the epithelial cells present include keratohyalin and bundles of cytoplasmic fibres.