Itchy bumps on elbows and knees. why?

Allerfy? Psoriaisis?: This could represent dermatitis or psoriasis. This could be assocated with food allergy or atopic dermatitis. Try hydrocrotisone over the counter. If not effective see dermatologist for evaluation amd treatment.

?irritation: Possibilities include atopic dermatitis, irritant dermatitis or possibly irritation due to friction depending on hobbies or occupation or bug bites. Although not typically limited to the joints, a condition called keratosis pilaris (excess keratin around hair follicle) is a consideration. If not resolve with moisturization or over the counter Hydrocortisone see a dermatologist.

Rash: Any skin condition is difficult to identify without seeing it or having more history. What does it look like?Is it red?Flad/raised? Etc. It could be that your knees have come into contact with something and is showing an allergic reaction. Could be eczema or psoriasis or skin rashes can sometimes be a sign of something going on internally. Best to have a doctor take a look and take a full history.

Many possibilities: There are many things that could cause the skin to itch. Several types of inflammatory conditions of the skin, allergic reactions, sun exposure, insect bites, and sometimes bacterial, viral or fungal infections can cause rashes on the skin. Have it evaluated by a professional and get the appropriate treatment.