Molecular basis of cellular immunity

Immunity can be termed as innate which involves immediate non specific actions such as physical barrier such as skin and mucous membranes as well as neutrophils and other leukocytes. Adaptive immunity is acquired throughout ones life time, its more specific but slower to respond and involves the response B and T lymphocytes, which are specific for specific invaders.

Pathogen recognition

In terms pathogen recognition, it is done by a group of cells called leukocytes. These are further divided into two categories:

1. Granulocytes: containing lysosome and specific granules (having different functions). They include neutrophils, basophils and eosinophils
2. Agranulocytes: lacking specific granules but may contain lysosomes. They include lymphocytes and monocytes.

Lymphocytes are specific immunity meaning a particular microbe will only be responded to by particular cells, while the rest of the cells mentioned above provide non specific immunity. Their functions are as listed below:

• Neutrophils: kill and phagocytose bacteria
• Eosinophils: kill helminthic and other parasites, modulate local inflammation
• Basophils: modulate inflammation, release histamine during allergy
• monocytes: precursors of macrophages, which phagocytose bacteria

Lymphocytes can be divided into B and T cells. B lymphocyte that recognizes the antigen of an invader, divide to form antibodies and memory B cells. The antibody, as well as other molecules stimulate helper T cells which have signaling functions and cytotoxic T cells which destroy the pathogen by causing its apoptosis. T cells also have memory cells.

Effector mechanisms

Effector mechanisms refers to the response that deals with the pathogen, whether to kill it neutralize it or cause its expulsion from the body. It consists of the cytotoxic T cells whose mechanism is described above, as well as that of antibodies.

Antibodies are glycoprotein molecules of the immunoglobulin family that are specific for certain antigens. They are released by plasma cells during an immune response. They act by neutralizing the microbe, precipitating any toxins released by the microbe, or agglutinating which clumps the microbes together making it easier for other immune cells to act on them.

MHC molecules

The major histocompatibility complex (MHC) are antigen-presenting proteins that are present on cells. There are two classes of them, class I and II. MHC class I bind a wide variety of proteosome derived peptide fragments representing proteins synthesized in the cell. This is sometimes referred to as the self antigen and helps the T cell recognized the cell as self. Class I is present on all nucleated cells. Some infected cells can thus also display fragments in class I that are non-self, helping the immune system eliminate these cells.

MHC class II is only found on antigen presenting cells and consists of foreign antigens that are left over from phagocytosis and such thus from the infected or abnormal cells the cell has digested. This signals T lymphocytes and activates the response against the source of these antigens.